[testrunner] explain how the testrunner progress percentage is calculated

Progress indicator calculates the percentage using the estimated amount of
tests.

When base tests produce more tests or when testrunner filters some tests, the
percentage terminates over 100% or under it.

This CL adds an informative message about how the percentage behaves.

R=machenbach@chromium.org
CC=yangguo@chromium.org,sergiyb@chromium.org

Bug: v8:8728
Change-Id: I91cafd2579ea1894ac347ff7483c307cd46c545d
Reviewed-on: https://chromium-review.googlesource.com/c/1477056Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59660}

tools/testrunner/standard_runner.py

@@ -320,6 +320,16 @@ class StandardTestRunner(base_runner.BaseTestRunner):
       for indicator in indicators:
         indicator.finished()
 
+
+      if tests.test_count_estimate:
+        percentage = float(results.total) / tests.test_count_estimate * 100
+      else:
+        percentage = 0
+
+      print ('>>> %d base tests produced %d tests (%d%s)'
+             ' non-filtered tests') % (
+          tests.test_count_estimate, results.total, percentage, '%')
+
       print '>>> %d tests ran' % (results.total - results.remaining)
 
       exit_code = utils.EXIT_CODE_PASS