test: SpannerRetryHelperTest is flaky

[olavloite]

On Windows/Java8 the following test failure sometimes occurs:

java.lang.AssertionError:

Expected: is <true>
     but: was <false>
	at com.google.cloud.spanner.SpannerRetryHelperTest.testExceptionWithRetryInfo(SpannerRetryHelperTest.java:219)