Fixes tests which fail when run in isolation

[wwestrop]

The following tests would fail with a null reference if they were run on their own:

• It_returns_the_state_that_was_found_first_Example_1()
• It_returns_the_state_that_was_found_first_Example_2()
• It_returns_the_state_that_was_found_first_Example_3()
• When_query_returns_false_It_raises_an_exception()

This is because SessionConfiguration is used early on in the tests, but it isn't initialised until BuildSession() is called later. When run in combination with other tests, they pass by chance, depending which other test has run before it, as in that case the SessionConfiguration is then initialised by another test.

I've added a SetUp method to ensure a fresh SessionConfiguration each test (it always uses a default one anyway). Alternatively we could swap around the order of the lines in these tests so that var session = BuildSession(....) comes before var state1 = ....