avocado.test: export environment variables to tests.

Export environment variables to the running test.
Really effective when using drop-in tests, that can make use of shell
variables "AVOCADO_*".

Example:

Export variable AVOCADO_VERSION with the current version of Avocado,
then the drop-in test can use $AVOCADO_VERSION within, and so on.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #128 from ruda/export_variables

avocado.test: export environment variables to tests.