tests: Import the backward compatibility API for all the tests.

This isn't very nice and pulls all sort of almost useless code into
the shims, making them less atomic. However the next few commits
will start another round of API standardization and the compatibility
layer wont be optional anymore.

tests/examples/shims/_common_template.lua

@@ -33,6 +33,8 @@ return function(_, _)
     -- backend.
     local bg = require("wibox.container.background")
     bg._use_fallback_algorithm()
+
+    require("awful._compat")
 end
 
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