text/template: add lock for Template.tmpl to fix data race

This adds a new lock protecting "tmpl".

This is a copy of https://golang.org/cl/257817 by Andreas Fleig,
updated for current tip, and updated to start running the
html/template TestEscapeRace test.

Thanks to @bep for providing the test case.

Fixes #39807

Change-Id: Ic8874484290283a49116812eeaffb8608346dc70
Reviewed-on: https://go-review.googlesource.com/c/go/+/316669


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>