Broaden the search for native test files.

Broaden the search for native test files.

Previously we were looking for test files with this pattern: test_*
I added *_test.[cc|cpp] and *_unittest.[cc|cpp]

The search also scan all the subdirectories of the build_path from
the test definition.

I added a filtering stage where missing tests are ignored.
For instance we may have a source file that has not been built for
the target, in which case it is ignored when we run the target tests.

In android_build.py I added 4 helper functions to get access to the
build environment:
- GetHostBin
- GetProductOut
- GetTargetSystemBin
- GetHostOsArch

Replace all the hardcoded linux-x86 strings with the value returned
by GetHostOsArch.