map test: reports partial result after an error (series & parallel)

[brodybits]

These test cases cover the effects of a for loop in initializeResults(), in lib/helpers.js.

Without this kind of a test case, it would be possible to remove the for loop (and an internal variable) without failing the test suite:

diff --git a/lib/helpers.js b/lib/helpers.js
index b0e913c..364c8c1 100644
--- a/lib/helpers.js
+++ b/lib/helpers.js
@@ -20,17 +20,8 @@ function defaultExtensions(extensions) {
 }
 
 function initializeResults(values) {
-  var keys = Object.keys(values);
   var results = Array.isArray(values) ? [] : {};
 
-  var idx = 0;
-  var length = keys.length;
-
-  for (idx = 0; idx < length; idx++) {
-    var key = keys[idx];
-    results[key] = undefined;
-  }
-
   return results;
 }

This is a code mutation that I discovered through mutation testing using Stryker Mutator, which I will propose in an upcoming PR.

For the parallel map API call, the test case waits for both the callback and remaining iterator to finish. A little tricky, hopefully not too fragile:)

[phated]

Thanks for this! I'm sorry it took so long to get to, but we are doing a pass at all the gulpjs modules.

I've rebased on the new CI and everything looks good. Cheers 🍻