[RFC][WIP] Add Unit Testing

[dwbuiten]

See the commit message below.

This is a rough draft of a test framework for FFMS2, based on googletest, based on an old initial patch by @lotharkript.

Some things to note:

• Currently it's just using wget to sync, and I've just shoved the sample file(s) up on a personal HTTP server. This will be replaced with either GCS hosting, or maybe somewhere that rsync can see. Opinions, please.
• I kinda half-integrated it with autotools; it uses a recursive Makefile call. If someone wants to integrate it "properly" with autotools, be my guest... googletest looks like it might be a PITA to do this for.
• Currently only one set of simple tests. More to be added after initial comments.
• Lacks MSVC project integration. I don't have a setup on-hand for this, and I am lazy...
• googletest is a git submodule. Opinions welcome on better/different ways.
• The gendata tool is not optimized, since it doesn't have to be, IMO.
• I chose a simple C++ file with const data for the "test data" format, since googletest is already C++, and I didn't feel like writing a parser... Options and opinons welcome.
• Do we want the test data (e.g. test.mp4.cpp) in the FFMS2 repo, or synced from a remote place, along with the test media files?

Please comment on anything and everything. This is rough.

Commit Message:

This adds support within the build system for googletest, as well
as an inital test. googletest is added as submodule. Tested to work
both in-tree and out-of-tree.

Run with:

    $ .../configure
    $ make test-sync # if needed
    $ make test

Commands may also have the following make variables set via command line:

    * SAMPLES_URL
    * SAMPLES_DIR

Example:
    $ make test-sync SAMPLES_DIR=/home/user/sampels SAMPLES_URL=https://localsamples.com/samples

Syncing currently relies on wget.

This does not currently include MSVC project integration.

A simple tool is included for generating the test data. It can be run
as, for example:

    $ cd $SAMPLES_DIR
    $ .../gendata sample.mp4 samplemp4data 0 > .../data/sample.mp4.cpp

This tool is not comprehensive, and is just a starting point. Sample data
may need to be manually generated for some problem samples.

CC: @lotharkript @isasi @tgoyne @myrsloik

[dwbuiten]

Ping.

[myrsloik]

I skimmed through and it looks reasonable

[dwbuiten]

Updated with parameterized tests, as per @lotharkript, along with version info in the test data, to help bisect FFmpeg if it happens to break (as it does).

Lots of other little bits fixed as per IRC convos.