Extending script queries to allow easier testing of warning messages

[fulghum]

This change lets you specify the expected number of warnings and an expected substring from test queries. It also tidies up some of the logic around running tests with expected warnings.

Example test syntax this allows you to write:

Assertions: []enginetest.ScriptTestAssertion{
	{
		Query:                           "select * from dolt_diff_t;",
		ExpectedWarning:                 1105,
		ExpectedWarningsCount:           4,
		ExpectedWarningMessageSubstring: "unable to coerce value from field",
		SkipResultsCheck:			true,
	},
},

Needed for: dolthub/dolt#2832

[zachmu]

LGTM, just one comment