[test] Fix subtest creation

Update flags before recalculating outcomes.

Bug: v8:6917
Change-Id: I5f54f7d14fd60f7e35a976e5200d3f7f0e74a5b7
Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/876364Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michał Majewski <majeski@google.com>
Cr-Commit-Position: refs/heads/master@{#50731}

tools/testrunner/objects/testcase.py

@@ -71,12 +71,12 @@ class TestCase(object):
     subtest.processor = processor
     subtest.procid += '.%s' % subtest_id
     subtest.keep_output = keep_output
+    if flags:
+      subtest.variant_flags = subtest.variant_flags + flags
     if variant is not None:
       assert self.variant is None
       subtest.variant = variant
       subtest._prepare_outcomes()
-    if flags:
-      subtest.variant_flags = subtest.variant_flags + flags
     return subtest
 
   def create_variant(self, variant, flags, procid_suffix=None):