Add test for yaml with no src dirs

tests/single-main/Main.hs

@@ -0,0 +1,6 @@
+module Main where
+
+import Lib
+
+main :: IO ()
+main = topReddit >>= print

tests/single-main/package.yaml

@@ -0,0 +1,4 @@
+name: single-main
+
+executable:
+    main: Main.hs

tests/single-main/test

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+# vim: ft=sh sw=2 et
+
+set -euo pipefail
+
+test() {
+  echo | $SNACK ghci
+  $SNACK run
+}
+
+SNACK="snack -j4 --package-yaml ./package.yaml" test