tests: Add test coverage for DirUtil.

Test: recovery_unit_test passes.
Change-Id: I764c56404c7ccdd57ae5486c946fbc9ac6ae7bc9
diff --git a/tests/Android.mk b/tests/Android.mk
index effed83..17e2b7f 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -33,6 +33,7 @@
 
 LOCAL_SRC_FILES := \
     unit/asn1_decoder_test.cpp \
+    unit/dirutil_test.cpp \
     unit/locale_test.cpp \
     unit/sysutil_test.cpp \
     unit/zip_test.cpp \