Add filesystem_test.go

It has only one test function as a starting point.

Bug: n/a
Test: m (soong tests)
Change-Id: I785b096805014a40dfd600f7baaf884f4016c23c
diff --git a/filesystem/Android.bp b/filesystem/Android.bp
index 791019d..3cdaa64 100644
--- a/filesystem/Android.bp
+++ b/filesystem/Android.bp
@@ -15,8 +15,10 @@
         "filesystem.go",
         "logical_partition.go",
         "vbmeta.go",
+        "testing.go",
     ],
     testSrcs: [
+        "filesystem_test.go",
     ],
     pluginFor: ["soong_build"],
 }