Add Tests for ProperIn and ProperContains

[brynrhodes]

Based on the outcome of this tracker: https://jira.hl7.org/browse/FHIR-57890

Add the following test cases for Interval ProperIn and ProperContains:

Interval[1, 5] properly includes 1 // true
Interval[1, 1] properly includes 1 // false
1 properly included in Interval[1, 5] // true
1 properly included in Interval[1, 1] // false
Interval[DateTime(2012, 1, 1), DateTime(2012, 1, 1)] properly includes DateTime(2012, 1, 1) // false
Interval[DateTime(2012, 1, 1), DateTime(2012, 1, 1)] properly includes DateTime(2012, 1, 1, 12, 0, 0, 0) // null, due to imprecise boundary comparisons