This change adds to the code-generation harness in asm_test.go support
for the use of a '$' placeholder name for test functions.

A few of uninformative function names are also changed to use the
placeholder, to confirm that the change works as expected.

Fixes #21500

Change-Id: Iba168bd85efc9822253305d003b06682cf8a6c5c
Reviewed-on: https://go-review.googlesource.com/57292
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>