[SPARK-29736][TESTS] Improve stability of tests for special datetime values

[MaxGekk]

What changes were proposed in this pull request?

• Retry the tests for special date-time values on failure. The tests can potentially fail when reference values were taken before midnight and test code resolves special values after midnight. The retry can guarantees that the tests run during the same day.
• Simplify getting of the current timestamp via Instant.now(). This should avoid any issues of converting current local datetime to an instance. For example, the same local time can be mapped to 2 instants when clocks are turned backward 1 hour on daylight saving date.
• Extract common code to SQLHelper
• Set the tested zoneId to the session time zone in DateTimeUtilsSuite.

Why are the changes needed?

To make the tests more stable.

Does this PR introduce any user-facing change?

No

How was this patch tested?

By existing test suites Date/TimestampFormatterSuite and DateTimeUtilsSuite.

[SparkQA]

Test build #113183 has finished for PR 26380 at commit 44aeda8.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• class DateTimeUtilsSuite extends SparkFunSuite with Matchers with SQLHelper

[MaxGekk]

@HyukjinKwon @cloud-fan Could you take a look at this.

1. This is rare case but could happen
2. This happened once I guess in [SPARK-29733][TESTS] Fix wrong order of parameters passed to assertEquals #26377 (comment)

[cloud-fan]

shall we set a threshold like 3? otherwise we may hit dead loop for real failures.

[MaxGekk]

Maybe I don't understand how such loop could happen but this is not recursive code. test can be invoke maximum 2 times.

[cloud-fan]

ah I mis-looked. It's not recursive. LGTM

[cloud-fan]

thanks, merging to master!