cmake: unity mode optimization for non-`CURLDEBUG` `testdeps` targets

cmake: unity mode optimization for non-`CURLDEBUG` `testdeps` targets

Include more sources in unity mode to optimize libtest and tests/server
builds for non-debug-enabled builds, syncing this pattern with `lib` and
`src`.

It reduces build steps from 62 to 47 (-14, -24%) with test bundles.
Without test bundles, from 680 to 642 (-38, -6%).

Follow-up to de0693f24943cd65f26a7b421a4304cbadb875a0 #16274
Follow-up to 3efba94f773db5d8ae19e33aa749ab7914cafeea #14765
Cherry-picked from #15000
Closes #16695