Tests: Avoid interference from environment in RunCMake.find_library

Clear $ENV{CMAKE_PREFIX_PATH} in several RunCMake.find_library tests
because on Windows, a CMAKE_PREFIX_PATH with two consecutive
semicolons causes a library to be found when it should not be.

None of the updated tests expect CMAKE_PREFIX_PATH from the environment
to influence the test, so clear the variable entirely to guarantee it
will not affect the test.

Fixes: #23111
Signed-off-by: William R. Dieter <william.r.dieter@intel.com>