Rename gdb test case file such that it starts by "gdb" like the other…

[JohanEngelen]

… gdb tests. (this enables disabling the test when gdb is not available on the test system)

Used in LDC's testsuite: https://github.com/ldc-developers/ldc/blob/40ad5f7583fe90a85fd675bfc0cfc287d565c94a/tests/dmd/tools/d_do_test.d#L1702-L1709

[dlang-bot]

Thanks for your pull request and interest in making D better, @JohanEngelen! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

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Testing this PR locally

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dub run digger -- build "stable + dmd#16458"