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.
This fixes few cases where driver eventor goroutines are leaked during
normal operations, but especially so in tests.
This change makes few modifications:
First, it switches drivers to use `Context`s to manage shutdown events.
Previously, it relied on callers invoking `.Shutdown()` function that is
specific to internal drivers only and require casting. Using `Contexts`
provide a consistent idiomatic way to manage lifecycle for both internal
and external drivers.
Also, I discovered few places where we don't clean up a temporary driver
instance in the plugin catalog code, where we dispense a driver to
inspect and validate the schema config without properly cleaning it up.
this allows us to drop a cyclical import, but is subobptimal as it
requires BaseDriver tests to move. This falls firmly into the realm of
being a hack. Alternatives welcome.
