[dev.typeparams] test: rename blank functions

This CL renames blank functions in the test/ directory so that they
don't rely on the compiler doing anything more than typechecking them.

In particular, I ran this search to find files that used blank
functions and methods:

$ git grep -l '^func.*\b_(' | xargs grep -n '^' | grep '\.go:1:' | grep -v '// errorcheck$'

I then skipped updating a few files:

* blank.go
* fixedbugs/issue11699.go
* fixedbugs/issue29870.go

  These tests specifically check that blank functions/methods work.

* interface/fail.go

  Not sure the motivation for the blank method here, but it's empty
  anyway.

* typeparam/tparam1.go

  Type-checking test, but uses "-G" (to use types2 instead of typecheck).

Updates #47446.

Change-Id: I9ec1714f499808768bd0dcd7ae6016fb2b078e5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/338094


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>