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

test/data/check-test-subdir/check-test-subdir.cabal

@@ -0,0 +1,15 @@
+name: check-test-subdir
+version: 0.1.0
+build-type: Simple
+cabal-version: >= 1.8
+
+library
+  build-depends: base == 4.*
+  hs-source-dirs: src
+  exposed-modules: Check.Test.Subdir
+
+test-suite test
+  type: exitcode-stdio-1.0
+  build-depends: base == 4.*
+  hs-source-dirs: test
+  main-is: Main.hs

test/data/check-test-subdir/src/Check/Test/Subdir.hs

@@ -0,0 +1,4 @@
+module Check.Test.Subdir (subdir) where
+
+subdir :: String
+subdir = "subdir"

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

@@ -0,0 +1,5 @@
+module Main where
+import Bar.Baz (baz)
+
+main :: IO ()
+main = putStrLn baz