misc/cgo/testcshared: add a c-shared test for android/arm.

- main3.c tests main.main is exported when compiled for GOOS=android.
- wait longer for main2.c (it's slow on android/arm)
- rearranged test.bash

Fixes #10070.

Change-Id: I6e5a98d1c5fae776afa54ecb5da633b59b269316
Reviewed-on: https://go-review.googlesource.com/9296


Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>