tests: Allow setting a subdir for output

tests: Allow setting a subdir for output

meson seems somewhat weak when it comes to handling
test output. We need to get the output from different
test runs into different locations, and the only
way to communicate from a test setup with the actual
test code seems the environment, so use that.

Make all tests that produce output in files respect
a TEST_OUTPUT_SUBDIR environment variable which specifies
the name of a subdirectory to use. This is combined
with the existing --output argument, which specifies
a per-test location.

Affected tests are reftests, css performance tests
and gsk compare tests.