include all GnuPG test suites that will run in Android

Each GnuPG project includes a tests/ folder with a bunch of test programs
and data that are normally run using `make check`.  Since building for
Android means cross-compiling, `make check` will not work.  That means we
have to do extra tricks to get the tests running on a device or emulator.

This also greatly increases the size of the tests, so they are no longer
included in the APK by default.  You must now manually run this command to
install them before building the APK:

make -C external assets-tests

closes #1270
REMOVE ME debug flags for native builds