runtime/metrics: simplify test to support more environments

go test sets the working directory to that of the package being tested,
so opening one of the package source files can be done in a simpler way.
This also allows the test to run in more environments, for example when
GOROOT_FINAL¹ is set.

Also remove the testenv.HasSrc-like check for Go source. The doc.go
file is a part of the package being built and tested, so it's expected
to be available. If it's important for this test to handle when a test
binary is built with go test -c and executed elsewhere without package
source files, something more than testenv.HasSrc would be needed.

¹ https://golang.org/cmd/go/#hdr-Environment_variables

Fixes #43085.

Change-Id: Ie6ade395a8fc7beebdadbad6f4873800138dfc26
Reviewed-on: https://go-review.googlesource.com/c/go/+/276452


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Trust: Dmitri Shuralyov <dmitshur@golang.org>