Set profiling environment variable when tests are run.

This has no effect most of the time, but if the runtime is built with
llvm-cov support, then this will cause each test to emit a .profraw
file, which can be merged with:

$ llvm-profdata merge -sparse Test/*.profraw -o libobjc.profdata

This can then be used to generate coverage reports from the test suite
and find code paths that are not currently being tested.

Test/CMakeLists.txt

@@ -81,7 +81,7 @@ function(addtest_flags TEST_NAME FLAGS TEST_SOURCE)
 		LINKER_LANGUAGE C
 	)
 	set_property(TEST ${TEST_NAME} PROPERTY
-		ENVIRONMENT "LD_LIBRARY_PATH="
+		ENVIRONMENT "LD_LIBRARY_PATH=" "LLVM_PROFILE_FILE=${TEST_NAME}.profraw"
 	)
 	target_link_libraries(${TEST_NAME} objc)
 endfunction(addtest_flags)