added a few tests for :pparse()

spec/pparse_spec.lua

@@ -0,0 +1,19 @@
+local Parser = require "argparse"
+
+describe("tests related to :pparse()", function()
+   it("returns true and result on success", function()
+      local parser = Parser()
+      parser:option "-s" "--server"
+      local ok, args = parser:pparse{"--server", "foo"}
+      assert.is_true(ok)
+      assert.same({server = "foo"}, args)
+   end)
+
+   it("returns false and bare error message on failure", function()
+      local parser = Parser()
+      parser:argument "foo"
+      local ok, errmsg = parser:pparse{}
+      assert.is_false(ok)
+      assert.equal("too few arguments", errmsg)
+   end)
+end)