CMake: fix a bug where only_validation-only_non_executable test configuration was running all tests

A nested loop was adding arguments specific to the current iteration to
a variable, where the arguments persisted into the next iteration.

The issue only affected the only_validation-only_non_executable
configuration, here's why.  The test subsets were configured in the
following order: primary, validation, all, only_validation, only_long.
The first three were not adding new arguments.  'all' was adding
arguments that were affecting 'only_validation'.  Then 'only_long' was
adding an argument of its own that was overriding the argument from
'all'.

Author: gribozavr

test/CMakeLists.txt

@@ -236,6 +236,10 @@ if(PYTHONINTERP_FOUND)
         endif()
 
         foreach(test_subset ${TEST_SUBSETS})
+          # We will add arguments specific for this subset to LIT_ARGS.  Copy
+          # the original value and restore it later.
+          set(LIT_ARGS_COPY "${LIT_ARGS}")
+
           set(want_this_combination TRUE)
           if(test_subset STREQUAL "only_long")
             # Long tests are only run in 'optimize_none' mode.
@@ -282,6 +286,7 @@ if(PYTHONINTERP_FOUND)
                 COMMENT "Running ${test_subset} Swift tests for ${VARIANT_TRIPLE}"
                 ${cmake_3_2_USES_TERMINAL})
           endif()
+          set(LIT_ARGS "${LIT_ARGS_COPY}")
         endforeach()
       endforeach()
     endforeach()