SpannerImplTest.testSpannerClosed test needs to detect if Spanner emulator is running

[odeke-em]

Coming here from my PR #3900 in which I've seen these consistent failures

Error:  com.google.cloud.spanner.SpannerImplTest.testSpannerClosed -- Time elapsed: 0.024 s <<< ERROR!

The test fails for both the emulator tests and multiplexed sessions too. We need guards to skip this test if an emulator isn't running or if it isn't, it needs to be running correctly.

Please fix this so that it doesn't cause CI to red and mislead contributors into thinking their code is wrong.