Template.escape makes the assumption that t.Lookup(t.Name()) is t
(escapeTemplate looks up the associated template by name and sets
escapeErr appropriately).

This assumption did not hold for a Cloned template, because the template
associated with t.Name() was a second copy of the original.

Add a test for the assumption that t.Lookup(t.Name()) == t.

One effect of this broken assumption was #16101: parallel Executes
racily accessed the template namespace because each Execute call saw
t.escapeErr == nil and re-escaped the template concurrently with read
accesses occurring outside the namespace mutex.

Add a test for this race.

Related to #12996 and CL 16104.

Fixes #16101

Change-Id: I59831d0847abbabb4ef9135f2912c6ce982f9837
Reviewed-on: https://go-review.googlesource.com/31092
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>