build: make test timeout configurable

The CMake test property TIMEOUT has higher priority than the
user-supplied --timeout switch. The exceptionally long default
setting (1000 seconds) can make tests that aren't entirely controlled
by test-monitor very painful to run.

This commit instead sets CTEST_TEST_TIMEOUT, allowing the user to
override the timeout at test run time.

CMakeLists.txt

@@ -1566,10 +1566,11 @@ if(BUILD_TESTS)
     set(TEST_MONITOR_DEFAULT_TIMEOUT 120)
   endif()
 
+  # The real timeouts are handled by test-monitor
+  set(CTEST_TEST_TIMEOUT 1000)
+
   function(configure_test test)
-    # The real timeouts are handled by test-monitor
-    set_tests_properties(${test}
-      PROPERTIES FAIL_REGULAR_EXPRESSION "FAILED" TIMEOUT 1000)
+    set_tests_properties(${test} PROPERTIES FAIL_REGULAR_EXPRESSION "FAILED")
   endfunction(configure_test)
 
   if(INSTALL_TESTSUITE)