test: Fix test log file name

[hebasto]

#19385 dropped .cpp infix (see #19385 (comment) and #19385 (comment)). However, src/test/README.md still refers to the foo_tests.cpp.log pattern:

bitcoin/src/test/README.md

Line 113 in e682e7d

`make check` will write to a log file `foo_tests.cpp.log` and display this file

This PR restores the pre-PR19385 behaviour, as appending is easier to implement than replacing when porting this functionality to CMake.

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[fanquake]

However, src/test/README.md still refer
This PR restores the pre-PR19385 behaviour,

Not sure this PR should be called a "fix"? As it seems the fix would be to just update the docs to match the current (intended) behaviour.

This instead seems to be a reversion to prior behaviour to facilitate something CMake related. I'm curious why this has only become an issue now, given we've already got the tests in CMake? Or is this something that was missing and is being ported now?

[maflcko]

Not sure about changing the test-only behavior here. Seems fine to just adjust the readme in the cmake pull, as the line has to be touched anyway. No need to be exact in porting this edge case?

[hebasto]

This instead seems to be a reversion to prior behaviour to facilitate something CMake related. I'm curious why this has only become an issue now, given we've already got the tests in CMake? Or is this something that was missing and is being ported now?

I had some concerns while documenting CTest behaviour in hebasto#329.