gh-127747: Fix w/o strace and with -Werror

A non-zero exit code occurs if the strace binary isn't found, and no
events will be parsed in that case (there is no output). Handle that
case by checking exit code before checking for events.

Still asserting around events rather than returning false, so that
hopefully if there's some change to `strace` that breaks the parsing,
will see that as a test failure rather than silently loosing strace
tests because they are auto-disabled.

Author: cmaloney

Lib/test/support/strace_helper.py

@@ -180,9 +180,10 @@ def get_syscalls(code, strace_flags, prelude="", cleanup="",
 @cache
 def _can_strace():
     res = strace_python("import sys; sys.exit(0)", [], check=False)
-    assert res.events(), "Should have parsed multiple calls"
-
-    return res.strace_returncode == 0 and res.python_returncode == 0
+    if res.strace_returncode == 0 and res.python_returncode == 0:
+        assert res.events(), "Should have parsed multiple calls"
+        return True
+    return False
 
 
 def requires_strace():