misc/cgo/testcshared: actually run test executable on android

CL 62593 broken TestExportedSymbols and TestUnexportedSymbols
because it started executing android test binary on host.
Make them run on android.

Hopefully fixes android build.

Change-Id: Ic0bb9f0cbbefca23828574282caa33a03ef72431
Reviewed-on: https://go-review.googlesource.com/62830


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>