AVOCADO_TEST_BASEDIR environment variable in exec-test

[richtja]

Is your feature request related to a problem? Please describe.
Since we are adding more spawners which spawn tasks in different environments, it might be difficult to determine the TEST_BASEDIR during testing. We have self.basedir and AVOCADO_TEST_BASEDIR variables available to avocado-instrumented tests but we miss this information on exec-test. Let's add AVOCADO_TEST_BASEDIR environment variable to exec-test which would point to the basedir on executed tests in testing environment.

Describe the solution you'd like
Same behavior of AVOCADO_TEST_BASEDIR on exec-test as it is on avocado-instrumented tests.