Add multi-column use-case tests

[azeque-art]

Add multi-column use-case tests for audit_missing, audit_stationarity and audit correlation_leakage. Closes #14.

[azeque-art]

Opened PR to address this issue

[imann128]

Thank you! This closes #44 well overall. Two things before merge:

• Can you change "Closes [TEST] Adding more types of tests for use cases #14" to "Closes [TEST] Multi-column coverage for each detector #44" in the description because [TEST] Adding more types of tests for use cases #14 is the broader parent issue and should not
  auto-close from this PR alone, as this PR, more precisely, targets the sub-issue [TEST] Multi-column coverage for each detector #44, so that would make it more accurate.

• test_healthcare_domain_threshold: "healthcare" isn't a supported domain (only "finance", "sensor", None), and missing_rate_threshold doesn't vary by domain at all; only cluster_threshold does. This test passes at 90% missing regardless of which of the four domains you pass it, so it isn't testing what the docstring claims.

You can either drop the domain= argument and rename it to something like test_high_missing_rate_flagged, or, if you want a genuine domain-comparison test, use "finance"/"sensor" and assert on cluster_threshold behaviour instead.