cmake: Make intrusive (formerly embedded) tests into a separate option

cmake: Make intrusive (formerly embedded) tests into a separate option

Previously, the CMake build enabled tests by default, and enabled both
modular and intrusive (embedded) tests with a single option. This is
a really bad idea if anyone is using CMake-built binaries in production.

DBUS_BUILD_TESTS now enables only the modular tests, which are safe to
enable in production builds.

A new DBUS_ENABLE_INTRUSIVE_TESTS option enables the intrusive test
instrumentation.

To preserve existing test coverage, explicitly enable the intrusive
tests in most CMake-based Gitlab-CI jobs (Debian native, openSUSE native,
Windows).

In jobs that have a mirrored pair of production/debug builds (openSUSE
and Debian mingw32/mingw64 cmake), instead we leave the production
build as-is and only build full test coverage in the debug build.

Co-authored-by: Philip Withnall <philip@tecnocode.co.uk>
Signed-off-by: Simon McVittie <smcv@collabora.com>