Fix test file naming scheme

spec/nextfile_spec.lua

@@ -0,0 +1,7 @@
+describe("nextfile", function()
+	local nextfile = require "nextfile"
+
+	it("returns true", function()
+		assert.is_true(nextfile.nextfile())
+	end)
+end)

tests/test-nextfile.lua

@@ -1,2 +0,0 @@
-describe("nextfile", function()
-end)