regression: rez-test Could not resolve to variant

[nerdvegas]

Regression first present in v2.84.0

Happens when rez-test has a test with run_on == 'pre_release':

13:44:12 WARNING  Could not resolve environment for this variant (/tmp/rez_testing_repo_5IOw7o/sequence/2.5.1/package.py[0]). This is a known issue and will be fixed once 'explicit variant selection' is added to rez.

...

Test results:
--------------------------------------------------------------------------------
0 succeeded, 0 failed, 1 skipped

Test  Status   Variant                                           Description
----  ------   -------                                           -----------
unit  skipped  /home/ajohns/rpackages/sequence/2.5.1/python-2.7  Could not resolve to variant (known issue)