Add extra assertion on number of warnings

Author: MadLittleMods

test/test.js

@@ -200,6 +200,7 @@ describe('postcss-css-variables', function() {
 						expect(colorDecl.value).to.be.a('string');
 						expect(colorDecl.value).to.be.equal('undefined');
 
+						expect(result.warnings().length).to.be.equal(1);
 						expect(result.warnings()[0].type).to.be.equal('warning');
 						expect(result.warnings()[0].text).to.be.equal('variable --missing is undefined and used without a fallback');
 					});