Matthew Woehlke authored Nov 20, 2017 and Brad King committed Nov 21, 2017

According to the documentation, tests can be discovered for a target
multiple times by using a different prefix and/or suffix to ensure name
uniqueness. However, while this worked for gtest_add_tests, it did not
work with gtest_discover_tests because the generated file that sets up
the tests was named based only on the target name, and so subsequent
discovery from the same target would clobber earlier discovery.

Fix this by introducing a counter that records how many times discovery
has been used on a target, and use this to generate unique names of the
generated test list files.