Fix test skipping

[graebm]

Issue:

The SKIP code worked in aws-crt-cpp's tests, not in any aws-c-* tests

Description of changes:

• Set SKIP_RETURN_CODE in CMake script that generates C test runner (the CPP script had it, but not the C script)
• Skip leak checks when a test is skipped
  • this keeps the tests simple, you can bail out in the middle without cleaning up

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.12%. Comparing base (ce899b9) to head (1b28e99).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1118 +/- ## ======================================= Coverage 83.12% 83.12% ======================================= Files 56 56 Lines 5754 5754 ======================================= Hits 4783 4783 Misses 971 971 ```

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