#19385 dropped .cpp
infix (see #19385 (comment) and #19385 (comment)). However, src/test/README.md
still refers to the foo_tests.cpp.log
pattern: https://github.com/bitcoin/bitcoin/blob/e682e7db7e399ce888e492eab8f99c389aae05b5/src/test/README.md?plain=1#L113
This PR restores the pre-PR19385 behaviour, as appending is easier to implement than replacing when porting this functionality to CMake.