Send SIGABRT to signal-test-app

[talzag]

Bug/issue #: rdar://90715477

Summary

Thread sanitizer doesn't handle SIGINT properly, and this causes the
signal-test-app to hang when tests are run under TSan. Sending SIGABRT
causes signal-test-app to exit under all circumstances.

Dependencies

None

Testing

Steps:

1. Open the package in Xcode
2. Select the SwiftDocC-Package scheme
3. Select Product > Scheme > Edit Scheme...
4. Select the Test row on the left side of the scheme editor window
5. Select the Diagnostics tab
6. Turn on the Thread Santizier
7. Select Product > Test
8. Verify that the tests complete, specifically that SignalTests.testTrappingSignal() doesn't hang.

Alternatively, run xcrun swift test --sanitize thread and verify that the test run completes.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[talzag]

@swift-ci Please test