custom matchers tests have inverted logic

[jiparis]

This relates to changes introduced in #2396: Go code might be correct, but tests have inverted logic and could be confusing. For example, this snipped in rego_test.go:

	t.Run("high severity matches medium expectation", func(t *testing.T) {
		matches, err := r.MatchesParameters(context.TODO(), policy,
			map[string]string{"severity": "high"},
			map[string]string{"severity": "medium"})
		require.NoError(t, err)
		assert.True(t, matches)
	})

It should match false, since the expectation is set to MEDIUM, but a policy evaluation with HIGH will never scan for MEDIUM results, hence it shouldn't match.