tests: keep strerr totally clean on successful runs

Debian's autopkgtest defaults to considering any output on stderr as a sign
that the tests failed.  it is simple to make this achieve that standard, so
it seems worth it for now at least.

tests/run-tests

@@ -112,7 +112,7 @@ fi
 
 # allow the location of aapt to be overridden
 if [ -z $aapt ]; then
-    aapt=`ls -1 $ANDROID_HOME/build-tools/*/aapt | sort | tail -1`
+    aapt=`ls -1 $ANDROID_HOME/build-tools/*/aapt 2> /dev/null | sort | tail -1`
 fi
 
 # try to use GNU sed on OSX/BSD cuz BSD sed sucks