[6.2] Add signal handler for SIGABRT_COMPAT on Windows. (#1059)

- **Explanation**: Make sure all public signals on Windows are handled
correctly in exit tests.
  - **Scope**: Windows exit tests
  - **Issues**: N/A
  - **Original PRs**: #1056
  - **Risk**: Low
  - **Testing**: CI jobs
  - **Reviewers**: @briancroom @stmontgomery @compnerd

Author: grynspan

Sources/Testing/ExitTests/ExitTest.swift

@@ -218,7 +218,7 @@ extension ExitTest {
     // exit code that is unlikely to be encountered "in the wild" and which
     // encodes the caught signal. Corresponding code in the parent process looks
     // for these special exit codes and translates them back to signals.
-    for sig in [SIGINT, SIGILL, SIGFPE, SIGSEGV, SIGTERM, SIGBREAK, SIGABRT] {
+    for sig in [SIGINT, SIGILL, SIGFPE, SIGSEGV, SIGTERM, SIGBREAK, SIGABRT, SIGABRT_COMPAT] {
       _ = signal(sig) { sig in
         _Exit(STATUS_SIGNAL_CAUGHT_BITS | sig)
       }