Chromium does not allow any third_party code to depend on base,
so the main test runner for swiftshader's tests in chromium can't
live inside the swiftshader repo. To solve this:

1) Added a test environment for swiftshader that will be used
for testing in chromium. Currently, it is simply empty.
2) Split unit tests in 2 files, unittests.cpp, which will
contain the tests ran by both chromium and swiftshader and
main.cpp, which will be used as the swiftshader test runner,
but NOT as the chromium test runner (that one will be added
in chromium at the same time I do the swiftshader update).
3) The BUILD.gn file was updated to make the chromium version
of the test depend on base and on the chromium version of the
swiftshader test runner (swiftshader_tests_main.cc, to be added).

Change-Id: I1128bb482565908bc285dcd6818d90d1d881a50e
Reviewed-on: https://swiftshader-review.googlesource.com/8409Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>