Fix test start time tracking (emscripten-core#25743)

Fix test start time tracking. Used when EMTEST_VISUALIZE=1 env. var. is
set.

Author: juj

test/parallel_testsuite.py

@@ -90,6 +90,7 @@ def printResult(res):
 
   olddir = os.getcwd()
   result = BufferedParallelTestResult()
+  result.start_time = start_time
   result.buffer = buffer
   temp_dir = tempfile.mkdtemp(prefix='emtest_')
   test.set_temp_dir(temp_dir)