Unit tests for formatting code, fix %%.

Also fix <signal.h> and <stdio.h> so they don't cause compiler warnings.

Change-Id: Ib1a746bf01de22d47dbd964de0e6af80a7c96303
diff --git a/tests/Android.mk b/tests/Android.mk
index 25da120..0685d4a 100644
--- a/tests/Android.mk
+++ b/tests/Android.mk
@@ -53,6 +53,7 @@
     -Werror \
 
 test_src_files = \
+    debug_format_test.cpp \
     dirent_test.cpp \
     fenv_test.cpp \
     getauxval_test.cpp \