misc/cgo/testcarchive: fix build

misc/cgo/testcarchive: fix build

I failed to rebase (and re-test) CL 21102 before submit, which meant
that two extra tests sneaked into testcarchive that still referenced
runtime.GOOS and runtime.GOARCH.

Convert the new tests.

While we're here, make sure pending tasks are flushed before running
the host tests. If not, the "##### misc/cgo/testcarchive" banner
and "PASS" won't show up in the all.bash output.

Change-Id: I41fc4ec9515f9a193fa052f7c31fac452153c897
Reviewed-on: https://go-review.googlesource.com/21106
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>