Test driver: normalise call stack line numbers into libraries

Ticket: #27387 (closed)

When comparing the stdout of tests that print out callstacks, we can't rely on the stability of exact line:column spans pointing into libraries (e.g. ghc-internal), as any change (such as adding a comment) can change them.

This MR addresses this by normalising away line:column in callstacks, but only when those point into internal libraries. We don't do this in general, as the exact span might be important to the test (e.g. for a span within the test module itself).