To ensure Review Bot works as expected, we need to write tests that

simulate a call to an external tool, and tests that actually execute an

external tool. A few of these have already been written, but maintenance

was already becoming problematic, as we want to ensure consistency

in both the data going into these tests and the data we assert.
This adds a new BaseToolTestCase class for tool tests, along with an

accompanying ToolTestCaseMetaclass and @integration_test and

@simulation_test decorators.
The decorators mark a test as being capable of integration testing and

simulation testing, respectively. They're intended for

BaseTool.execute() unit tests. The metaclass will scan for these and

split any test function into separate simulation/integration tests (if

the appropriate decorators are applied).
This keeps things very easy to maintain, but with the expense of a quirk

with test names. A test_foo() defined on the class will no longer be

available to the test runner. Instead, separate

test_simulation_foo() and test_integration_foo() test functions will

be available. This isn't immediately obvious, but is a quirk we can

learn to live with.

Made use of this in upcoming changes. Tests all passed.