[2.62] 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
Signed-off-by: Simon McVittie smcv@collabora.com

(cherry picked from commit 42d8e177)