fix spawning of yarn process

Author: SimenB

lib/package-manager/test.js

@@ -65,12 +65,13 @@ function test(packageManager, context, next) {
       }
     }
 
-    /* Run `npm test`, or `/path/to/customTest.js` if the customTest option
+    /* Run `npm/yarn test`, or `/path/to/customTest.js` if the customTest option
      was passed */
-    const args = context.options.customTest ?
-      [context.options.customTest] : [packageManagerBin, 'test'];
-
-    const proc = spawn(nodeBin, args, options);
+    const proc = context.options.customTest
+      ? spawn(nodeBin, [context.options.customTest], options)
+      // We spawn the package manager binary directly instead of via `node` t
+      // support it being a script instead of a JS file
+      : spawn(packageManagerBin, ['test'], options);
     const finish = timeout(context, proc, next, 'Test');
 
     proc.stdout.on('data', function (data) {