Tests/RunCMake: Allow tests to control build tree behavior

Teach the run_cmake to allow tests to set a custom test build directory.
Also add an option to skip removing the build directory.

Tests/RunCMake/RunCMake.cmake

@@ -26,8 +26,12 @@ function(run_cmake test)
     endif()
   endforeach()
   set(RunCMake_TEST_SOURCE_DIR "${top_src}")
-  set(RunCMake_TEST_BINARY_DIR "${top_bin}/${test}-build")
-  file(REMOVE_RECURSE "${RunCMake_TEST_BINARY_DIR}")
+  if(NOT RunCMake_TEST_BINARY_DIR)
+    set(RunCMake_TEST_BINARY_DIR "${top_bin}/${test}-build")
+  endif()
+  if(NOT RunCMake_TEST_NO_CLEAN)
+    file(REMOVE_RECURSE "${RunCMake_TEST_BINARY_DIR}")
+  endif()
   file(MAKE_DIRECTORY "${RunCMake_TEST_BINARY_DIR}")
   if(NOT DEFINED RunCMake_TEST_OPTIONS)
     set(RunCMake_TEST_OPTIONS "")