Test data directory requirement is always needed: testing enabled or not

[Fuad-HH]

The main CMakeLists.txt always includes testing.cmake whether testing enabled or not, which is not ideal.

pcms/CMakeLists.txt

Line 141 in af3d7d9

include(test/testing.cmake)

And it requires the PCMS_TEST_DATA_DIR :

pcms/test/testing.cmake

Lines 6 to 14 in af3d7d9

	# TODO use submodule FetchContent/ExternalProject/ExternalData
	set(PCMS_TEST_DATA_DIR
	""
	CACHE PATH "Path to a local copy of the pcms_coupling_data repo.")
	if(NOT EXISTS ${PCMS_TEST_DATA_DIR})
	message(
	FATAL_ERROR "PCMS_TEST_DATA_DIR \"${PCMS_TEST_DATA_DIR}\" is not accessible"
	)
	endif()