test: update test/run.go and some tests to use importcfg

test: update test/run.go and some tests to use importcfg

Using importcfg instead of depending on the existence of .a files for
standard library packages will enable us to remove the .a files in a
future cl.

Change-Id: I6108384224508bc37d82fd990fc4a8649222502c
Reviewed-on: https://go-review.googlesource.com/c/go/+/440222
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>