GoogleTest: Restore support for list arguments in TEST_EXECUTOR

Refactoring in commit 889a7146ff (GoogleTestAddTests: Refactor into
callable method, 2020-03-16, v3.18.0-rc1~450^2~3) accidentally
parsed `TEST_EXECUTOR` as a single-value argument instead of a list.

Modules/GoogleTestAddTests.cmake

@@ -44,8 +44,8 @@ function(gtest_discover_tests_impl)
   cmake_parse_arguments(
     ""
     ""
-    "NO_PRETTY_TYPES;NO_PRETTY_VALUES;TEST_EXECUTABLE;TEST_EXECUTOR;TEST_WORKING_DIR;TEST_PREFIX;TEST_SUFFIX;TEST_LIST;CTEST_FILE;TEST_DISCOVERY_TIMEOUT;TEST_XML_OUTPUT_DIR"
-    "TEST_EXTRA_ARGS;TEST_PROPERTIES"
+    "NO_PRETTY_TYPES;NO_PRETTY_VALUES;TEST_EXECUTABLE;TEST_WORKING_DIR;TEST_PREFIX;TEST_SUFFIX;TEST_LIST;CTEST_FILE;TEST_DISCOVERY_TIMEOUT;TEST_XML_OUTPUT_DIR"
+    "TEST_EXTRA_ARGS;TEST_PROPERTIES;TEST_EXECUTOR"
     ${ARGN}
   )