converted the existing testPlanState() from terraform.State to
states.State to fix various plan tests.

reverted the "bandaid" in plans/planfile/tfplan.go - at this moment the
backend tests do not include backend configuration, and so the planfile
package can write the plan file but not read it back in. That will be
revisted in a separate track of work.

I have no confidence in the change to plans/planfile/tfplan.go. The
tests were passing an empty backend config, which planfile  was able to
write to a file but not read from the same file. This change let me move
past that and it did not break any tests in the planfile package, but I
am concerned that it introduces undesired behavior.

for TestLocal_applyBackendFail, config is relative to the tests' wd

The legacy test wasn't really testing anything useful, just the plan
behavior with no diff in a module returned a module with no diff.
There's no reason to convert this to the new plans, since there's no
legacy behavior to match.

a plan with no diff should return proposed state

The "internal" provisioners are still run in a separate process, and
need to be updated to restart on each walk.

Some call diagnostics were being missed

While the schema Diff fucntion returns a nil diff when creating an empty
(except for id) resource, the Apply function expects the diff to be
initialized and ampty.

PlanResourceChange isn't returning the diff, but rather it is returning
the destired state. If the propsed state results in a nil diff, then,
the propsed state is what should be returned.

Make sure Meta fields are not nil, as the schema package expects those
to be initialised.

check responses for nil values

plugin processes

Modify the plugin factories to create a new plugin process for each
individual plugin.

While providers shouldn't return nil values, we need to protect against
them. Always assign valid null cty values when the response contains a
nil.

The test suite was not updated to deal with the new assumptions of the
HCL2-based expression evaluator, which requires configuration to exist
as well as state. Therefore we add a simple configuration fixture to have
it validate expressions against.

This also includes updates to expect the different error messages that the
new evaluator produces.

Logs will be hidden unless verbose mode is enabled on the test run.

This was inadvertently lost in the consoliation of EvalReadDataDiff and
EvalReadDataApply into a single EvalReadData.

Data resources get processed during refresh, so it doesn't make sense to
have a data resource test that doesn't run the refresh walk.

In the reorganization of the data source read code we missed the fact that
the plan phase must _always_ generate a read data diff, never directly
read, because the state generated during plan is throwaway.

This only matters in the -refresh=false case, since normally refresh has
already taken care of this, but that is still an important case, covered
by the TestContext2Apply_dataBasic test.

The underlying bug has now been fixed,
in b3b62429 .

This is not normally considered valid, but since this is a test helper
we will allow it and just treat it as an empty body.

We were calling from PersistState into RefreshState, but RefreshState is
protected by the same lock as PersistState and so the call would deadlock.

Instead, we introduce a new entry point refreshState which can be used
when already holding the lock.

For historical reasons sometimes we have nil state in situations where
we'd still like to persist state snapshots to a store. To make life easier
for those callers, we'll substitute an empty state if we are given a nil
one, thus allowing us to still generate a valid serialization that will
load back in as an empty state.

(this is a WIP prototype)

Various things drifted since these tests were originally written. This
catches them up to the latest implementations of state decoding,
upgrading, etc.

An earlier change made the tests not compile here. We now need to use the
legacyPluginMap function, since pluginMap has now been replaced with
helpers to produce new-style plugin _sets_.

An earlier change introduced a new function testConfig to the main code
for this package, which conflicted with a function of the same name in
the test code.

Here we rename the function from the test code, allowing for the more
generally-named testConfig to be the one in the main code.

terraform.RootModulePath is no longer present, but
addrs.RootModuleInstance is equivalent to it.

The test provider comes with a lot of baggage since it's designed to be
used as a plugin, so instead we'll just use the mock provider
implementation directly, and so we can (in a later commit) configure it
appropriately for what our tests need here.