Fold *systems-being-defined* into the *asdf-cache*.

Add regression test for no infinite loop
when processing asd files that mutually define each other's systems.
(Prompted by Robert Strandh inquiring about the bug fixed in 2.015.[23]
after my mentioning an infinite loop in my ASDF3 article.)
Checked that removing the set-asdf-cache-entry in parse-defsystem triggers the bug.