[test] Fix basic tests to to actually cover weakCompareExchange

[lorentey]

Oops, we had two separate tests for compareExchange and weakCompareExchange, but they were both invoking compareExchange only.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've verified that my change does not break any existing tests.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test