All embedder unit-tests have to setup the Flutter project arguments from scratch
before launching the engine. The boilerplate and having to deal with the low
level C API during each engine launch is a hinderance to writing tests.

This patch introduces an EmbedderTest fixture that sets up all the embedder side snapshots before allowing the unit test to create a FlutterConfigBuilder` that
the test can use to incrementally build and edit the Flutter project
configuration. From the given state state of a configuration, multiple engines
can be launched with their lifecylces managed by appropriate RAII wrappers.

This allows the a fully configured Flutter engine to be launched using 4 lines
of code in a fixture.

```
EmbedderConfigBuilder builder;
builder.SetSoftwareRendererConfig();
builder.SetAssetsPathFromFixture(this);
builder.SetSnapshotsFromFixture(this);
auto engine = builder.LaunchEngine();
```