fix: read time should be used for transaction reads

[danieljbruce]

Right now, read calls with a transaction do not use the provided snapshot read time. The code change in src ensures that read options always gets passed along to the Gapic layer.

A separate unit test file was created for the test because all of the current unit test files have mocks that we don't want to use. Mocking out functions in the Gapic layer is becoming a common use case so we should create a file that allows us to mock out runQuery for each test and write a test there with the intention of writing more tests there in the future.