Move finder_test filesystem helper functions to fs/test.go

Move the filesystem helper functions used by finder_test.go into the
fs package so they can also be used by fs tests.

Bug: 157656545
Test: m checkbuild
Change-Id: I5ac3a1f502d8e2cb7c15dd70ca85a46d20f939a6
diff --git a/finder/fs/Android.bp b/finder/fs/Android.bp
index 27e3c7d..835eb0b 100644
--- a/finder/fs/Android.bp
+++ b/finder/fs/Android.bp
@@ -22,6 +22,7 @@
     srcs: [
         "fs.go",
         "readdir.go",
+        "test.go",
     ],
     testSrcs: [
         "readdir_test.go",