Always build tests if we enabled installed-tests
If we're cross-compiling, the installed-tests are useful even if we can't run them on the build machine: we can copy them to the host machine (possibly via a distro package like Debian's libglib2.0-tests) and run them there.
While I'm changing the build-tests condition anyway, deduplicate it.
Based on a patch by Helmut Grohne.
Bug-Debian: https://bugs.debian.org/941509
This regressed in e7b0d89a, so please cherry-pick to 2.62 branch if accepted.