test: validate 'expected' argument to mustCall()

instead of silently overwriting invalid values with the default

PR-URL: nodejs#10692
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/common.js

@@ -398,7 +398,10 @@ function runCallChecks(exitCode) {
 
 
 exports.mustCall = function(fn, expected) {
-  if (typeof expected !== 'number') expected = 1;
+  if (expected === undefined)
+    expected = 1;
+  else if (typeof expected !== 'number')
+    throw new TypeError(`Invalid expected value: ${expected}`);
 
   const context = {
     expected: expected,

test/parallel/test-common.js

@@ -5,3 +5,11 @@ var assert = require('assert');
 common.globalCheck = false;
 global.gc = 42;  // Not a valid global unless --expose_gc is set.
 assert.deepStrictEqual(common.leakedGlobals(), ['gc']);
+
+assert.throws(function() {
+  common.mustCall(function() {}, 'foo');
+}, /^TypeError: Invalid expected value: foo$/);
+
+assert.throws(function() {
+  common.mustCall(function() {}, /foo/);
+}, /^TypeError: Invalid expected value: \/foo\/$/);