html/template: add tests from text/template

Copy and adapt tests from text/template, to exercise more of html/template's copy.

Various differences in behavior are flagged with NOTE comments or t.Skip
and documented in #40075. Many of them are probably bugs.
One clarifying test case added to both text/template and html/template.

No changes to the package itself.

Change-Id: Ifefad83d647db846040d24c2741a0244b00ade82
Reviewed-on: https://go-review.googlesource.com/c/go/+/241084


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>