Bug 1649440 - Move ContentTaskUtils into specialpowers to make SpecialPowers.spawn work in reftests. r=jmaher

We'd like to use SpecialPowers.spawn to do something in cross-origin iframes in reftests.
Without moving the file calling SpecialPowers.spawn in reftests raises an exception
at here [1] in SpecialPowersChild.sys.mjs.

[1] https://searchfox.org/mozilla-central/rev/2bf90dc51ce7e8274ce208fbb9d68b3ff535185e/testing/specialpowers/content/SpecialPowersChild.sys.mjs#1637

Depends on D184439

Differential Revision: https://phabricator.services.mozilla.com/D185246

testing/mochitest/BrowserTestUtils/moz.build

@@ -12,5 +12,4 @@ TESTING_JS_MODULES += [
     "ContentEventListenerChild.sys.mjs",
     "ContentEventListenerParent.sys.mjs",
     "ContentTask.sys.mjs",
-    "ContentTaskUtils.sys.mjs",
 ]

testing/specialpowers/moz.build

@@ -21,6 +21,7 @@ FINAL_TARGET_FILES.content += [
 ]
 
 modules = [
+    "content/ContentTaskUtils.sys.mjs",
     "content/MockColorPicker.sys.mjs",
     "content/MockFilePicker.sys.mjs",
     "content/MockPermissionPrompt.sys.mjs",