meson: define tests

meson: define tests

Tests can be run with 'ninja-build test' or using 'mesontest'.
'-Dtests=unsafe' can be used to include the "unsafe" tests in the
test suite, same as with autotools.

v2:
- use more conf.get guards are optional components
- declare deps on generated headers for test-{af,arphrd,cap}-list

v3:
- define environment for tests

  Most test don't need this, but to be consistent with autotools-based build, and
  to avoid questions which tests need it and which don't, set the same environment
  for all tests.

v4:
- rework test generation

  Use a list of lists to define each test. This way we can reduce the
  boilerplate somewhat, although the test listings are still pretty verbose. We
  can also move the definitions of the tests to the subdirs. Unfortunately some
  subdirs are included earlier than some of the libraries that test binaries
  are linked to.  So just dump all definitions of all tests that cannot be
  defined earlier into src/test. The `executable` definitions are still at the
  top level, so the binaries are compiled into the build root.

v5:
- tag test-dnssec-complex as manual

v6:
- fix HAVE_LIBZ typo
- add missing libgobject/libgio defs
- mark test-qcow2 as manual