cmd/go: respect gcflags, ldflags in 'go test'

Fixes bug introduced by https://golang.org/cl/129059 where
gcflags='all=...' and ldflags='all=...' would not be applied to some
packages built by 'go test'.

LoadImport used to set gcflags/ldflags for the Package objects it
created, in https://golang.org/cl/129059 this code was factored out to
setToolFlags. The codepath of `go build` was updated to call
setToolFlags appropriatley, but the codepath of `go test -c` wasn't,
resulting in gcflags/ldflags being applied inconsistently when building
tests.

This commit changes TestPackagesFor to call setToolFlags on the package
objects it creates.

Fixes #27681

Change-Id: Idcbec0c989ee96ec066207184611f08818873e8d
Reviewed-on: https://go-review.googlesource.com/c/136275


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>