On non-debug builds we might end up with an empty list of tests
when generating the schedule. On older cmake versions < 3.14
trying to sort an empty list will produce an error, so
we check for empty list here.
To support tests with different configuration options, we split the
tests into *test configurations*. Each test configuration NAME will have
- A configuration template file `NAME.conf.in` that is used to run the
suite of tests.
- A variable `TEST_FILES_<NAME>` listing the test files available for
that test suite.
- A variable `SOLO_TESTS_<NAME>` that lists the tests that need to be
run as solo tests.
The code to generate test schedules is then factored out into a
separate file and used for each configuration.
