The string from get_test_name() can be used for writing screenshot files and
others. Starting the name with the fixture number makes an alphabetized listing
of output files look unorganized.

Let's change the test name to begin with the test (source) name with fixture
and element numbers as suffixes. That makes a file listing easier to look
through, when you have multiple tests each saving multiple screenshot files.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>