Three of these tests were still using the compiler selected for building
CMake itself. In general our test suite is supposed to let each test
select the compiler from the environment. This is particularly
important on macOS where CMake might be built with compilers inside
Xcode, which require an explicit `CMAKE_OSX_SYSROOT`.
CMake 3.27 deprecates compatibility with CMake < 3.5. Update tests that
do not cover older interfaces to avoid the deprecation warning.
Follow the pattern from:
* commit 7b07ccdd2b (Tests/*Only: Update cmake_minimum_required versions,
2020-06-15, v3.19.0-rc1~629^2~1)
* commit 72e7c45e98 (Tests: Bump CMake minimum required in tests to 2.8.12,
2020-12-22, v3.20.0-rc1~224^2)
* commit f6b4db365a (Tests: bump cmake_minimum_required version to 2.8.12,
2021-04-04, v3.21.0-rc1~372^2)
Also remove explicit `cmake_policy` settings made redundant by the
version.
Modify the launcher test to also try to build two executables, having
respectively build and link errors. This is intended to test that
launchers are used when running compile and link commands, as well as
custom commands. (In particular, this should catch breakage such as that
fixed by ce71bd9505a.)
Brad King