Call abort() instead of exit() for convenience when debugging.

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 Change: 182495
 ServerID: perforce.ravenbrook.com

mps/code/testlib.c

@@ -328,7 +328,7 @@ void verror(const char *format, va_list args)
   vfprintf(stderr, format, args);
   fprintf(stderr, "\n");
   fflush(stderr); /* make sure the message is output */
-  exit(1);
+  abort();
 }