text/template: add error check for parenthesized first argument in pipeline
An error check was missing: If the first argument of a pipeline is
parenthesized, and the pipeline has further arguments, then
syntactically the pipeline is a function invocation and there must
be a "call". Tricky rare corner case, but easily caught.
Add the error check and some tests to verify behavior.
Fixes #31810.
Change-Id: Ica80b7c11284e4ea9e8cc94a01dbbc9a67e42079
Reviewed-on: https://go-review.googlesource.com/c/go/+/206124 Reviewed-by: Ian Lance Taylor <iant@golang.org>