Signed-off-by: Yoan Blanc <yoan@dosimple.ch>

Signed-off-by: Yoan Blanc <yoan@dosimple.ch>

Copy the updated version of freeport (sdk/freeport), and tweak it for use
in Nomad tests. This means staying below port 10000 to avoid conflicts with
the lib/freeport that is still transitively used by the old version of
consul that we vendor. Also provide implementations to find ephemeral ports
of macOS and Windows environments.

Ports acquired through freeport are supposed to be returned to freeport,
which this change now also introduces. Many tests are modified to include
calls to a cleanup function for Server objects.

This should help quite a bit with some flakey tests, but not all of them.
Our port problems will not go away completely until we upgrade our vendor
version of consul. With Go modules, we'll probably do a 'replace' to swap
out other copies of freeport with the one now in 'nomad/helper/freeport'.

Fix a bug where a millicious user can access or manipulate an alloc in a
namespace they don't have access to.  The allocation endpoints perform
ACL checks against the request namespace, not the allocation namespace,
and performs the allocation lookup independently from namespaces.

Here, we check that the requested can access the alloc namespace
regardless of the declared request namespace.

Ideally, we'd enforce that the declared request namespace matches
the actual allocation namespace.  Unfortunately, we haven't documented
alloc endpoints as namespaced functions; we suspect starting to enforce
this will be very disruptive and inappropriate for a nomad point
release.  As such, we maintain current behavior that doesn't require
passing the proper namespace in request.  A future major release may
start enforcing checking declared namespace.

In a job registration request, ensure that the request namespace "header" and job
namespace field match.  This should be the case already in prod, as http
handlers ensures that the values match [1].

This mitigates bugs that exploit bugs where we may check a value but act
on another, resulting into bypassing ACL system.

[1] https://github.com/hashicorp/nomad/blob/v0.9.5/command/agent/job_endpoint.go#L415-L418

Close is invoked in a different goroutine from test

The default job here contains some exec task config (for setting
command and args) that aren't used for mock driver.  Now, the alloc
runner seems stricter about validating fields and errors on unexpected
fields.

Updating configs in tests so we can have an explicit task config
whenever driver is set explicitly.

Although the really exciting change is making WaitForRunning return the
allocations that it started. This should cut down test boilerplate
significantly.

"Ask forgiveness, not permission."

Instead of peaking at TaskStates (which are no longer updated on the
AR.Alloc() view of the world) to only read logs for running tasks, just
try to read the logs and improve the error handling if they don't exist.

This should make log streaming less dependent on AR/TR behavior.

Also fixed a race where the log streamer could exit before reading an
error. This caused no logs or errors to be displayed sometimes when an
error occurred.