Omit spurious string from unit test debugging efforts

Omit spurious string from unit test debugging efforts

When both 'broken' and 'failed' test cases appear in unit test output

...
===> Broken tests
lib/isc/tests/socket_test:main  ->  broken: Test case timed out  [300.022s]
===> Failed tests
lib/isc/tests/time_test:main  ->  failed: 2 of 6 tests failed  [0.006s]
===> Summary
...

spurious '===>' string gets matched, that results in the following
error:

  Usage error for command debug: '===>' is not a test case identifier (missing ':'?).

Following change makes sure the string is omitted.

I checked on FreeBSD and OpenBSD that the AWK construct is supported.

(cherry picked from commit 9e6f6156f7fcd6713531960582d5393d05ac569d)