Add makesingular to test_src

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.

test/test_src.lua

@@ -71,6 +71,30 @@ local tests = {
       loch = "lochs",
       tech = "techs",
    },
+   makesingular = {
+      algae = "alga",
+      amoebae = "amoeba",
+      baluchitheria = "baluchitherium",
+      Bordeaux = "Bordeaux",
+      buses = "bus",
+      dingoes = "dingo",
+      ["gateaux au chocolat"] = "gateau au chocolat",
+      larvae = "larva",
+      lice = "louse",
+      matzot = "matzo",
+      mycelia = "mycelium",
+      nemeses = "nemesis",
+      oxen = "ox",
+      potatoes = "potato",
+      ["pots of tulips"] = "pot of tulips",
+      priestesses = "priestess",
+      ronin = "ronin",
+      ["set of dragon scales"] = "set of dragon scales",
+      techs = "tech",
+      tuna = "tuna",
+      vertebrae = "vertebra",
+      vortices = "vortex",
+   },
    an = {
       a = "an a",
       b = "a b",