fix: test_sec_audit: ignore cargo audit output

Since we run this test twice in case of a PR, cargo audit locks the
database and one of the processes may print something (even with -q
specified) in stdout (‽). I believe this should be printed in stderr,
but we can workaround it here easily.

    warning: directory /usr/local/rust/advisory-db is locked
    {...}

Use `grep` to filter out the extraneous output.

Signed-off-by: Pablo Barbáchano <pablob@amazon.com>

Author: pb8o

tests/integration_tests/security/test_sec_audit.py

@@ -35,6 +35,6 @@ def set_of_vulnerabilities(output: CommandReturn):
         )
 
     git_ab_test_host_command_if_pr(
-        "cargo audit --deny warnings -q --json",
+        "cargo audit --deny warnings -q --json |grep -Po '{.*}'",
         comparator=set_did_not_grow_comparator(set_of_vulnerabilities),
     )