regression: only print start-of-test messages if stdout is a tty.

This avoids clutter in non-tty outputs, such as regression emails.

misc/regression/run_tests.py

@@ -220,6 +220,7 @@ def run_test(test, status_queue, verbose=False):
 def print_test_line_start(test_name, legacy=False):
     if legacy:
         return
+    if sys.stdout.isatty():
         print("  Started %-25s " % (test_name + " ..."))
         sys.stdout.flush()