Let's add a check that teststmpdir is shared across different jobs when
set manually via environment variables. This also contains a little
improvement in one of the used tests.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

The "AVOCADO_TESTS_COMMON_TMPDIR" is an environmental variable name used
to store the location of a common tmpdir. To avoid issues with
typos/name changes let's use the variable in `avocado.core.test` to
share the env variable name everywhere.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

This commit creates a job pre/post plugin that generates a temporary
directory intended to be consumed by the tests. The temporary directory
will be created in 'pre', available via environment variable and removed
in 'post. We are also exposing the tests temporary directory in the Test
API.

Reference: https://trello.com/c/P1JOtVY7Signed-off-by: NAmador Pahim <apahim@redhat.com>