Update Spanner Interval integration tests to call `UsingEmulator()`

The Spanner integrations tests have been written to expect particular behavior from the emulator, even it is an error, rather than just skipping the test altogether. See all the other UsingEmulator() calls in this file, for example. This allows us to learn when the emulator changes behavior and react accordingly, rather than just forgetting about the whole issue forever more.

[It also helped the emulator team, who (at least previously) ran our tests to check their implementation. Rather than just seeing successes no matter what they did, expecting the old (error) behavior allowed them to verify that things had indeed changed, and also to compare their new behavior to that of the service.]

Find the Spanner Interval integration test file and inspect the existing UsingEmulator() calls nearby. Update the Interval integration tests to use the same pattern, retaining expectations for emulator errors rather than skipping those tests. Run the relevant Spanner integration test target and confirm the emulator behavior remains explicitly checked.

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