Stop using ctest

[CaseyCarter]

It's pointless to use ctest to run one of two different LIT commands when we can simply run LIT directly. This will make it easier to add future tests: we can simply add more targets, we won't need to jump through hoops to tell ctest not to run the new test all the time. This change will ease adding a new EDG-only test target for the IntelliSense team.

Detailed changes:

• In tests/CMakeLists.txt:
  • Replace add_test targets with add_custom_targets. The stl and stlasan tests are now test and extra-asan-tests targets, respectively.
  • Replace list(APPEND unset_variable_name a b c d) with the equivalent set(unset_variable_name a b c d) which makes it clear that we expect unset_variable_name to be undefined.
  • Merge ${Python_EXECUTABLE} into STL_LIT_COMMAND, and extract ${STL_LIT_TEST_DIRS}. The makes it easy to reuse ${STL_LIT_COMMAND} and ${STL_LIT_TEST_DIRS} with target-specific LIT options.
  • Note that USES_TERMINAL in the add_custom_targets makes ninja run those targets serially, maintaining the observable effects of the removed set_tests_properties(stl stlasan PROPERTIES RUN_SERIAL ON).
• Remove enable_testing from the top-level CMakeLists.txt; it does nothing now that we have no add_tests. We still use BUILD_TESTING to control including the test targets in the generated build system, but I'm not certain there's a good reason to do so. Maybe to enable folks without python to build but not test the STL?
• In run-tests.yml, replace ctestOptions parameter with a testTargets parameter, a list of targets to build via cmake --build meow --target woof quack. This needs to be set in asan-pipeline.yml and plumbed through build-and-test.yml with a default of test to run the vanilla STL tests.
• Remove/Replace the ctest discussion in README.md.
• Drive-by: Report that the psutil python module is unavailable at most once per stl-lit run. Seeing three repetitions in the CI logs bothered me.

[CaseyCarter]

• /azp run STL-ASan-CI (There's a later push in the PR, but it's a non-functional change to documentation so I didn't revalidate.)

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

🛑 😻 💚