cmd/compile: additional test cleanup

Refactoring to make it slightly easier to add tests,
easier to add variable-printing-support for Delve,
and made naming and tagging more consistent.

No changes to the content of the test itself or when it is
run.

Change-Id: I374815b65a203bd43b27edebd90b859466d1c33b
Reviewed-on: https://go-review.googlesource.com/84979
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/compile/internal/ssa/testdata/i22600.dlv-dbg-race.nexts

+  ./testdata/i22600.go
+8:	func test() {
+9:		pwd, err := os.Getwd()
+10:		if err != nil {
+14:		fmt.Println(pwd)
+15:	}
+19:	}