There are already pretty solid tests of makeplural, so add some in the
other direction, since it's similarly complicated and has to implement
many of the same special cases and exceptions in the other direction. I
originally tried just running the makeplural tests backwards to this
end, but several of them are tests to guarantee already-plural words
aren't further pluralized, so are not reversible.
These plurals (e.g. "gateau" -> "gateaux") were added to makeplural and
makesingular in 0c0262e, so the test was outdated and failed. I also
added another test for "Federal Bureau of Investigation", since 'bureau'
is an exception to the -eau rule (pluralizes as 'bureaus') and the
exception was originally implemented in a way that would have caused the
"Federal Bureau of Investigation" test to fail -- so it seems like a
good pitfall to test for.
