Fix flaky test that verifies secret push protection timeout

What does this MR do and why?

Problem

When RSpec's main process terminates and attempts to clean up child processes upon completion, it terminates subprocesses where the scans might be still ongoing. This behavior is not recognized by the upstream library (parallel), which manages all forked subprocesses it created for running scans. When the upstream library attempts to close its forked subprocesses which already terminated, it raises an ArgumentError with the message bad signal type NilClass, resulting in flaky failures in the test expectations.

Here's an example CI job that fails infrequently: https://gitlab.com/gitlab-org/gitlab/-/jobs/6935051992

Solution

Since the above problem occurs only in the test pipeline, I've rescued the problem-causing error in the spec.

Relevant Issue Numbers

• Flaky spec/lib/gitlab/secret_detection/scan_spe... (#439614 - closed) • Vishwa Bhat • 17.2

MR acceptance checklist

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