On android the generated header files are located in
pkg/$(go env GOOS)_$(go env GOARCH)_testcshared.
The test was broken since https://go-review.googlesource.com/9798.

The installation path differs based on codegenArgs
(around src/cmd/go/build.go line 389), and the codegenArgs
is platform dependent.

Change-Id: I01ae9cb957fb7676e399f3b8c067f24c5bd20b9d
Reviewed-on: https://go-review.googlesource.com/9980Reviewed-by: Ian Lance Taylor <iant@golang.org>