Symlinking busybox may fail when the test code and the test temporary
directory live on different volumes/partitions; so we should copy
instead.  This situation arises in the Vagrant setup, where the code
repository live on special file sharing volume.

Somewhat unrelated, remove `f.Sync()` invocation from a test copyFile
helper function.  Sync is useful only for crash recovery, and isn't
necessary in our test setup.  The sync invocation is a significant
overhead as it requires the OS to flush any cached writes to disk.