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_target`s. 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_target`s 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_test`s. 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.