Add slashes in test coverage examples to satisfy re2 syntax

What does this MR do?

Current documentation provides test coverage examples to use in CI jobs to determine test coverage. The examples no longer worked as the re2 syntax requires pre- and appending slashes.

Modified examples have been validated for syntax errors through the pipeline validator https://gitlab.com/ThomasTNO/gitlab/-/ci/editor?branch_name=master&tab=2

Resolves the config must be a regular expression with re2 syntax error at pipelines. error message.

Related issues

This closes #369092

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