[spec] Better insulation (#4972)

Some combination of Travis and/or older Busted necessitated insufficient insulation.

Follow-up to https://github.com/koreader/koreader/pull/4970

Space condensing carried over; reset at the start of readertoc.

spec/unit/commonrequire.lua

@@ -1,3 +1,14 @@
+-- don't try to overwrite metatables so we can use --auto-insulate-tests
+-- shamelessly copied from https://github.com/Olivine-Labs/busted/commit/db6d8b4be8fd099ab387efeb8232cfd905912abb
+local ffi = require "ffi"
+local old_metatype = ffi.metatype
+local exists = {}
+ffi.metatype = function(def, mttable)
+    if exists[def] then return exists[def] end
+    exists[def] = old_metatype(def, mttable)
+    return exists[def]
+end
+
 require "defaults"
 package.path = "?.lua;common/?.lua;rocks/share/lua/5.1/?.lua;frontend/?.lua;" .. package.path
 package.cpath = "?.so;common/?.so;/usr/lib/lua/?.so;rocks/lib/lua/5.1/?.so;" .. package.cpath