travis: Add tests for builtin printf hook implementation

We can't test Vstr as it does not properly handle negative int arguments
for custom format callbacks, so some of the enum tests would fail.

scripts/test.sh

@@ -13,6 +13,8 @@ DEPS="libgmp-dev"
 
 case "$TEST" in
 default)
+	# should be the default, but lets make sure
+	CONFIG="--with-printf-hooks=glibc"
 	;;
 openssl)
 	CONFIG="--disable-defaults --enable-tools --enable-openssl"
@@ -22,6 +24,9 @@ gcrypt)
 	CONFIG="--disable-defaults --enable-tools --enable-gcrypt --enable-pkcs1"
 	DEPS="libgcrypt11-dev"
 	;;
+printf-builtin)
+	CONFIG="--with-printf-hooks=builtin"
+	;;
 all)
 	CONFIG="--enable-all --disable-android-dns --disable-android-log
 			--disable-dumm --disable-kernel-pfroute --disable-keychain
@@ -29,8 +34,6 @@ all)
 			--disable-osx-attr --disable-tkm --disable-uci"
 	# not enabled on the build server
 	CONFIG="$CONFIG --disable-af-alg"
-	# TODO: add tests for different printf implementations?
-	CONFIG="$CONFIG --disable-vstr"
 	# TODO: enable? perhaps via coveralls.io (cpp-coveralls)?
 	CONFIG="$CONFIG --disable-coverage"
 	DEPS="$DEPS libcurl4-gnutls-dev libsoup2.4-dev libunbound-dev libldns-dev