Lots of tests + table schema support for SQL & MySQL

[NickCraver]

This PR:

• Adds tests for the .Shared and .MySQL libraries
• Adds testing for .MySQL
• Adds tests for all storage providers in an easy-to-extend way
• Adds config loading and overrides (e.g. environmental variables in AppVeyor)
• Adds overloads to SQLErrorStore and MySQLErrorStore to specify the table used
• Adds TestConfig.json support

Next up is Appveyor support via SQL and MySQL services.

[NickCraver]

Appveyor is green and running all storage tests, using the schema overloads for random test table names. Score! Merging in.