Add support test script

[lorentey]

This semi-automated test script exercises some build configurations that aren't covered by regular CI runs.

This includes:

• Building and testing with various swiftpm setups:
  • debug vs release,
  • with and without doublewide atomics,
  • with and without library evolution turned on (this is unsupported but why break it),
  • warnings-as-errors on & off
• Building & testing with xcodebuild
  • macOS
  • Mac Catalyst
  • iOS
  • watchOS
  • tvOS
• Building & testing with CMake
  • debug vs release

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

[lorentey]

An issue @glessard noticed is that the spm.debug tests are taking a huge amount of time on AS machines. This is worth investigating, as it seems to indicate some ARM-specific codegen issue. Double wide atomics are inordinately slow (or even broken) in debug builds.

[glessard]

We found issues in debug-mode tests on M1, but those are not caused by this fine script.

[lorentey]

We'll use #41 to track the arm64 issue; this looks good to merge.

[lorentey]

@swift-ci test