GoogleTest: Rename TIMEOUT parameter to avoid clash

In gtest_discover_tests(), the TIMEOUT keyword was making it
impossible to set the TIMEOUT test property via the PROPERTIES
keyword. This would be a frequent case, but it doesn't complain
and instead silently does something different to what would
normally be expected. The TIMEOUT keyword has been renamed
to DISCOVERY_TIMEOUT, thereby removing the clash.

This is a breaking change. 3.10.1 and 3.10.2 were the only versions
that supported the TIMEOUT keyword and uses of it were likely
not working as intended.

Fixes: #17801

Modules/GoogleTest.cmake

@@ -150,6 +150,7 @@ same as the Google Test name (i.e. ``suite.testcase``); see also
                          [NO_PRETTY_TYPES] [NO_PRETTY_VALUES]
                          [PROPERTIES name1 value1...]
                          [TEST_LIST var]
+                         [DISCOVERY_TIMEOUT seconds]
     )
 
   ``gtest_discover_tests`` sets up a post-build command on the test executable
@@ -217,7 +218,7 @@ same as the Google Test name (i.e. ``suite.testcase``); see also
     executable is being used in multiple calls to ``gtest_discover_tests()``.
     Note that this variable is only available in CTest.
 
-  ``TIMEOUT num``
+  ``DISCOVERY_TIMEOUT num``
     Specifies how long (in seconds) CMake will wait for the test to enumerate
     available tests.  If the test takes longer than this, discovery (and your
     build) will fail.  Most test executables will enumerate their tests very
@@ -225,6 +226,16 @@ same as the Google Test name (i.e. ``suite.testcase``); see also
     longer timeout.  The default is 5.  See also the ``TIMEOUT`` option of
     :command:`execute_process`.
 
+    .. note::
+
+      In CMake versions 3.10.1 and 3.10.2, this option was called ``TIMEOUT``.
+      This clashed with the ``TIMEOUT`` test property, which is one of the
+      common properties that would be set with the ``PROPERTIES`` keyword,
+      usually leading to legal but unintended behavior.  The keyword was
+      changed to ``DISCOVERY_TIMEOUT`` in CMake 3.10.3 to address this
+      problem.  The ambiguous behavior of the ``TIMEOUT`` keyword in 3.10.1
+      and 3.10.2 has not been preserved.
+
 #]=======================================================================]
 
 #------------------------------------------------------------------------------
@@ -357,7 +368,7 @@ function(gtest_discover_tests TARGET)
   cmake_parse_arguments(
     ""
     "NO_PRETTY_TYPES;NO_PRETTY_VALUES"
-    "TEST_PREFIX;TEST_SUFFIX;WORKING_DIRECTORY;TEST_LIST;TIMEOUT"
+    "TEST_PREFIX;TEST_SUFFIX;WORKING_DIRECTORY;TEST_LIST;DISCOVERY_TIMEOUT"
     "EXTRA_ARGS;PROPERTIES"
     ${ARGN}
   )
@@ -368,8 +379,8 @@ function(gtest_discover_tests TARGET)
   if(NOT _TEST_LIST)
     set(_TEST_LIST ${TARGET}_TESTS)
   endif()
-  if(NOT _TIMEOUT)
-    set(_TIMEOUT 5)
+  if(NOT _DISCOVERY_TIMEOUT)
+    set(_DISCOVERY_TIMEOUT 5)
   endif()
 
   get_property(
@@ -418,7 +429,7 @@ function(gtest_discover_tests TARGET)
             -D "NO_PRETTY_VALUES=${_NO_PRETTY_VALUES}"
             -D "TEST_LIST=${_TEST_LIST}"
             -D "CTEST_FILE=${ctest_tests_file}"
-            -D "TEST_DISCOVERY_TIMEOUT=${_TIMEOUT}"
+            -D "TEST_DISCOVERY_TIMEOUT=${_DISCOVERY_TIMEOUT}"
             -P "${_GOOGLETEST_DISCOVER_TESTS_SCRIPT}"
     VERBATIM
   )