On Valgrind bots, print a message every 20 minutes

I believe that the timeout failure on the Valgrind bot occurs
because we skip many tests consecutively without printing any
output.  Skipping these tests requires non-trivial work.

Printing a message every 20 minutes will avoid timeouts.

BUG=skia:4740
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1554193002

Review URL: https://codereview.chromium.org/1554193002
4 files changed