tests/: cavern.tst testcase back2 is now actually used.  Fix a bug
in this testcase, and extend it to cover a variant of the situation
reported as a bug in therion by Bill Gee to the therion list.
tests/inferplumbs.svx: Remove unused file containing the start of a
testcase for "*infer plumbs" - this isn't useful as we already test
this in cmd_infer.svx.
tests/Makefile.am: Ship some missing .out files and compare.tst.