Write test for spoofed navigator* values in workers
legacy/trac#13027 (moved) fixed the problem with navigator* values not being spoofed. Back then we could not write a Mozmill test as Mozmill itself was affected by the underlying issue (https://bugzilla.mozilla.org/show_bug.cgi?id=1062920 together with https://bugzilla.mozilla.org/show_bug.cgi?id=1078163).