Tests: Record dates by instant in the drop-in snapshot

* DropInSnapshot.j:
  - Describe a CPDate by its UTC instant instead of its class name.
  - Record the string value of a control holding a date by the date's instant, since the browser formats it in the local time zone.

* DropInArchives.json: Update the CPDatePicker entry to the instant it archives.

* DropInArchiveTest.j: State that the test must pass in every time zone, and how to check it in UTC.

Tests/AppKit/DropInArchiveTest.j

@@ -125,6 +125,14 @@ var fs = require("fs"),
 
 /*
     An archive written by main must restore the view it was written from.
+
+    The descriptions compared must be the same on every machine. A date, and the
+    string value of a control that holds one, are recorded by the date's instant in
+    UTC, because their text is in the local time zone (DropInSnapshot.j). GitHub's
+    continuous integration runs this test in UTC, so run it in UTC as well as in the
+    local time zone before committing a change:
+
+        TZ=UTC npx jake test
 */
 - (void)testExistingArchivesRestoreTheOriginalView
 {