Tests: don't create byte-code cache when calling Python from SVN

Creating `__pycache__` directories in SVN's lib/ directory can cause
updating SVN to fail. Add the -B flag when TEST_PYTHON_EXE from LIBDIR
is used so so Python doesn't generate this cache.

tests/python/CMakeLists.txt

@@ -41,7 +41,7 @@ function(add_python_test testname testscript)
 
   add_test(
     NAME ${testname}
-    COMMAND ${TEST_PYTHON_EXE} ${testscript} ${ARGN}
+    COMMAND ${TEST_PYTHON_EXE} ${TEST_PYTHON_EXE_EXTRA_ARGS} ${testscript} ${ARGN}
   )
   set_tests_properties(${testname} PROPERTIES ENVIRONMENT
     LSAN_OPTIONS=exitcode=0:$ENV{LSAN_OPTIONS}