Currently in tests, we have calls to os.Remove and os.RemoveAll where we
don't check the returned error. This hides useful error messages when
tests fail to run, such as "too many open files".
This change checks for more filesystem related errors and calls t.Fatal
if there is an error.
Currently, temporary test files and directories are written in lots of
different paths within the OS's temp dir. This makes it hard to know
which files are from nats-server and which are unrelated. This in turn
makes it hard to clean up nats-server test files.
Staticcheck has probably been updated and is finding new errors.
They have been fixed.
Also, moved the run of staticcheck before running the test suite,
so if it fails, it fails sooner ;-).
