This adds Mongoose as a third-party library, and builds on top of that
an embedded HTTP server for our unit tests.
We are using a fork of Mongoose pending the merging of this PR:
https://github.com/cesanta/mongoose/pull/2301
The embedded web server will make our HTTP-based tests independent of
httpbin.org, which will make them quick and reliable.
Rewrite test_data_util.cc to locate files relative to the source file
itself, rather than using a service from chromium `base::`.
Issue #1047 (CMake porting)
Issue #346 (absl porting)