text/template: indirect interfaces before slicing

text/template: indirect interfaces before slicing

The recently added slice function used indirectInterface, but then
forgot to actually call reflect.Value.Slice on its result. Calling the
Slice method on the original Value without indirectInterface would
result in a panic, if our slice was indeed behind an interface.

Fix that, and add test cases for all three built-in functions that work
with slices.

Fixes #36199.

Change-Id: I9a18f4f604a3b29967eefeb573f8960000936b88
Reviewed-on: https://go-review.googlesource.com/c/go/+/211877
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>