Resolve this warning:
".../Tests/CMakeCommands/target_link_libraries/depB.cpp", line 8: warning:
variable "a" was declared but never referenced
DepA a;
^
The CMakeCommands.build_command test performs output/error checking
so move it over to RunCMake to re-use the generalized infrastrucure.
This is the only test left using Tests/CMakeCommands/CMakeLists.txt
so remove it.
Add a "CMakeCommands.find_package" test to run CMake on a bunch of cases
in which find_package fails. Check that the process return code and
warning/error messages are as expected. Record expected test output in
corresponding files for reference by the check. These files will also
serve as a reference for the message text in each case.