CTest: Fix timeout when grandchild keeps pipes open

When a test's process creates its own child and exits, the grandchild
may keep pipes open.  Fix CTest logic to correctly timeout if the
grandchild does not exit and close the pipes before the timeout expires.
This was broken by commit b5e21d7d2e (CTest: Re-implement test process
handling using libuv, 2017-12-10, v3.11.0-rc1~117^2) which added an
unnecessary condition to the timeout handling.

Fixes: #20116

Source/CTest/cmProcess.cxx

@@ -278,9 +278,6 @@ void cmProcess::OnTimeoutCB(uv_timer_t* timer)
 
 void cmProcess::OnTimeout()
 {
-  if (this->ProcessState != cmProcess::State::Executing) {
-    return;
-  }
   this->ProcessState = cmProcess::State::Expired;
   bool const was_still_reading = !this->ReadHandleClosed;
   if (!this->ReadHandleClosed) {

Tests/RunCMake/CTestTimeout/Fork-stdout.txt

@@ -0,0 +1,6 @@
+Test project [^
+]*/Tests/RunCMake/CTestTimeout/Fork-build
+    Start 1: TestTimeout
+1/1 Test #1: TestTimeout ......................\*\*\*Timeout +[0-9.]+ sec
++
+0% tests passed, 1 tests failed out of 1

Tests/RunCMake/CTestTimeout/RunCMakeTest.cmake

@@ -12,3 +12,11 @@ function(run_ctest_timeout CASE_NAME)
 endfunction()
 
 run_ctest_timeout(Basic)
+
+if(UNIX)
+  string(CONCAT CASE_CMAKELISTS_SUFFIX_CODE [[
+    target_compile_definitions(TestTimeout PRIVATE FORK)
+]])
+  run_ctest_timeout(Fork)
+  unset(CASE_CMAKELISTS_SUFFIX_CODE)
+endif()

Tests/RunCMake/CTestTimeout/TestTimeout.c

@@ -8,6 +8,13 @@
 
 int main(void)
 {
+#ifdef FORK
+  pid_t pid = fork();
+  if (pid != 0) {
+    return 0;
+  }
+#endif
+
 #if defined(_WIN32)
   Sleep((TIMEOUT + 4) * 1000);
 #else