Refactor BUILD.gn so can easily be embedded in other projects

1.
BUILD.gn: Don't use the extra Chromium clang warnings

Also removes the unused .gn secondary_sources.

2.

Move fuzzers in test/ instead of testing/

This frees up testing/ to be the git subtree of Chromium's src/testing/
that contains test.gni, gtest, gmock and libfuzzer

3.

DEPS: get the whole testing/ subtree of Chromium

4.

BUILD.gn: Simplify the standalone gtest targets

These targets definitions are inspired from ANGLE's and add a variable
that is the path of the googletest directory so that it can be made
overridable in future commits.

6.

BUILD.gn: Add overridable variables for deps dirs

This avoids hardcoded paths to dependencies that make it hard to
integrate SPIRV-Tools in other GN projects.