Added a single test function script and fix debug-test.sh to be more robust

[mofosyne]

Continuation of https://github.com/ggerganov/llama.cpp/pull/7096 somewhat to address cases where a person may just want to run a single test.

Also discovered that the test binaries expects to be run in the git root context so need to fix debug-test.sh to take that into account.

@josh-ramer you may want to have a quick look a this to see if this would also be useful to you as well.

This is what the output visually looks like. With emphasis on the last three lines for clearer feedback

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(Created this to try and more quickly pin down a CI issue)