Gather all hs-source-dirs to check test modules in sub-directories

test/data/check-test-subdir/test/Bar/Baz.hs

@@ -0,0 +1,5 @@
+module Bar.Baz (baz) where
+import Foo (foo)
+
+baz :: String
+baz = unwords [foo, "baz"]

test/data/check-test-subdir/test/Foo.hs

@@ -0,0 +1,3 @@
+module Foo (foo) where
+
+foo = "foo"

test/data/check-test-subdir/test/Main.hs

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+module Main where
+import Bar.Baz (baz)
+
+main :: IO ()
+main = putStrLn baz