cmake: Rework tests target for Coverage configuration and multi-config generators

[hebasto]

When using a multi-config generator, such as "Ninja Multi-Config", the build system still builds the tests binary for the "Coverage" configuration, which is meaningless:

$ cmake -B build -G "Ninja Multi-Config"
$ cmake --build build --config Coverage
[18/19] Building C object src/CMakeFiles/tests.dir/Coverage/tests.c.o
/home/hebasto/git/secp256k1/secp256k1/src/tests.c:18:13: note: ‘#pragma message: Defining VERIFY for tests being built for coverage analysis support is meaningless.’
   18 |     #pragma message("Defining VERIFY for tests being built for coverage analysis support is meaningless.")
      |             ^~~~~~~
[19/19] Linking C executable src/Coverage/tests

This PR fixes the issue.

It is an alternative to #1251 and #1291.

[real-or-random]

@hebasto Can you summarize what this PR does and why is it preferable to #1291? The approach in #1291 seemed natural to me.

[hebasto]

Rebased and reworked.

The following is an example workflow on Ubuntu 24.04:

cmake -B build -DCMAKE_GENERATOR="Ninja Multi-Config"
cmake --build build --config Coverage  # Ninja uses all available cores by default
ctest --test-dir build -j $(nproc) -C Coverage
mkdir -p build/coverage
gcovr --merge-mode-functions=separate --exclude 'src/bench*' --html --html-details -o build/coverage/coverage.html
firefox build/coverage/coverage.html

[hebasto]

@hebasto Can you summarize what this PR does and why is it preferable to #1291? The approach in #1291 seemed natural to me.

Answered in #1716 (comment).