Fix Date_Add.ion test and add tests for Date_diff.ion function

[xd1313113]

Multiple issues with Date_Add and Date_Diff conformance tests.

date_add valid cases{time_part:year,quantity:1,timestamp:"2017T",result:(success 2018T)}, compileOption: PERMISSIVE
PRuntimeException{error=PError{code=INTERNAL_ERROR, severity=ERROR, kind=SYNTAX, location=null, properties={CAUSE=org.partiql.parser.PartiQLParserException: Expected one of: 0, 1, 2, 3, 4, 5, 6, 7}}}

java.lang.IllegalArgumentException: invalid timestamp: missing local offset: "2018-01-02T03:04"
	at com.amazon.ion.Timestamp.fail(Timestamp.java:921)
	at com.amazon.ion.Timestamp.valueOf(Timestamp.java:1127)
	at com.amazon.ionelement.api.Ion.ionTimestamp(Ion.kt:161)
	at com.amazon.ionelement.api.Ion.ionTimestamp$default(Ion.kt:159)
	at org.partiql.runner.util.DatumUtilKt.toIonElement(DatumUtil.kt:33)
	at org.partiql.runner.executor.EvalExecutor.toIon(EvalExecutor.kt:93)
	at org.partiql.runner.executor.EvalExecutor.toIon(EvalExecutor.kt:37)
	at org.partiql.runner.test.TestRunner.run(TestRunner.kt:57)
	at org.partiql.runner.test.TestRunner.access$run(TestRunner.kt:11)
	at org.partiql.runner.test.TestRunner$test$1.invoke(TestRunner.kt:17)
	at org.partiql.runner.test.TestRunner$test$1.invoke(TestRunner.kt:14)
	at org.partiql.runner.test.TestRunner.runSkipped(TestRunner.kt:34)
	at org.partiql.runner.test.TestRunner.test(TestRunner.kt:14)
	at org.partiql.runner.ConformanceTestBase.validatePartiQLEvalTestData(ConformanceTestBase.kt:30)