SCOV/instr.py: correctly detect warnings when running gnatcov

gnatcov warnings (and low warnings) are prefixed with '***'. Check for
such patterns in gnatcov output when warnings_as_errors is true.

Change-Id: I002a071479f6e7a06f059b6d5353948d21c8c36a
TN: V615-017

Author: Jugst3r

testsuite/SCOV/instr.py

@@ -112,8 +112,9 @@ def xcov_instrument(gprsw, covlevel, extra_args=[], dump_trigger="auto",
 
     if warnings_as_errors:
         output = contents_of(out)
+
         thistest.fail_if(
-            "!!!" in output,
+            "***" in output,
             f"Warnings detected in the output of 'gnatcov instrument':"
             f"\n{indent(output)}"
         )