corrects tests for timestamp with nanosecond precision

[desaikd]

Description of changes:
This PR works on modifying tests for timestamps with nanoseconds precision to resolve test failure in amazon-ion/ion-hash-go#71 that bumps ion-go to v1.1.1.

For example a timestamp: 2021-05-27 00:19:05.000050000 has coefficient: 50000 and exponent: 9.
Inside encodeTimeDate function, we get the correct coefficient by calling time.Nanosecond which returns 50000 however the exponent is calculated by converting the coefficient to string and getting the length which returns 5 and is incorrect. The fix is to set the exponent to 9 if there are nanoseconds because time.time will always contain 9 precision units.

Changes:

• Adds exponent as 9 in the binary timestamp value.

Related change: amazon-ion/ion-go#177

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[zslayton]

Per offline discussion, these tests actually caught a bug in ion-go (amazon-ion/ion-go#177).

[desaikd]

This change isn't required anymore as there was a bug in ion-go which is fixed with v1.1.2