html/template: revert "avoid race when escaping updates template"

This reverts CLs 274450 and 279492, except for the new tests.
The new race test is changed to skip, as it now fails.
We can try again for 1.17.

Original CL descriptions:

    html/template: attach functions to namespace

    The text/template functions are stored in a data structure shared by
    all related templates, so do the same with the original, unwrapped,
    functions on the html/template side.

    html/template: avoid race when escaping updates template

For #39807
Fixes #43855

Change-Id: I2ce91321ada06ea496a982aefe170eb5af9ba847
Reviewed-on: https://go-review.googlesource.com/c/go/+/285957


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>