test: increase coverage of process.emitWarning

Previously our tests did not check these codepaths as seen at
coverage.nodejs.org

PR-URL: https://github.com/nodejs/node/pull/9556
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-process-emitwarning.js

@@ -10,10 +10,12 @@ process.on('warning', common.mustCall((warning) => {
   assert(warning);
   assert(/^(Warning|CustomWarning)/.test(warning.name));
   assert(warning.message, 'A Warning');
-}, 3));
+}, 7));
 
 process.emitWarning('A Warning');
 process.emitWarning('A Warning', 'CustomWarning');
+process.emitWarning('A Warning', CustomWarning);
+process.emitWarning('A Warning', 'CustomWarning', CustomWarning);
 
 function CustomWarning() {
   Error.call(this);
@@ -24,6 +26,16 @@ function CustomWarning() {
 util.inherits(CustomWarning, Error);
 process.emitWarning(new CustomWarning());
 
+const warningNoToString = new CustomWarning();
+warningNoToString.toString = null;
+process.emitWarning(warningNoToString);
+
+const warningThrowToString = new CustomWarning();
+warningThrowToString.toString = function() {
+  throw new Error('invalid toString');
+};
+process.emitWarning(warningThrowToString);
+
 // TypeError is thrown on invalid output
 assert.throws(() => process.emitWarning(1), TypeError);
 assert.throws(() => process.emitWarning({}), TypeError);