[Aidl-Tests] Add some unit tests and infrastructure for XAT.Aidl.

[jpobst]

Context: #5726

We have someone willing to contribute fixes for
Xamarin.Android.Tools.Aidl.dll (yay!). However, we no longer have
any expertise with AIDL available on the team, and there are no unit
tests for this area. This makes us very reluctant to make changes
which could potentially break the currently working aspects of AIDL.

Add a new tests/Xamarin.Android.Tools.Aidl-Tests unit test project
and seed it with a few examples from our repo and a few large examples
from other OSS repos.

Note: this does not mean that the behavior tested by these tests is
necessarily correct. It simply reflects what the code does today,
in order that we can see any changes made by future PR's.

~~ Note ~~

The format for test data files is delimited by 4 pound signs as:

<input>

####

<output>

Test results can be generated by:

• Placing the input in a test file
• Enabling the GENERATE_OUTPUT define at the top of
  AidlCompilerTestBase.cs
• Running the test
• The test file in bin/TestData will now contain the test output
  which can be copied back to the original test data file.