Add gdb script to test mbedtls_zeroize()

The gdb script loads the programs/test/zeroize program and feeds it as
imput its own source code. Then sets a breakpoint just before the last
program's return code and checks that every element in memory was
zeroized. Otherwise it signals a failure and terminates.

The test was added to all.sh.
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 497a261..81ab2ca 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -421,7 +421,7 @@
 # Make sure the tools we need are available.
 check_tools "$OPENSSL" "$OPENSSL_LEGACY" "$GNUTLS_CLI" "$GNUTLS_SERV" \
             "$GNUTLS_LEGACY_CLI" "$GNUTLS_LEGACY_SERV" "doxygen" "dot" \
-            "arm-none-eabi-gcc" "i686-w64-mingw32-gcc"
+            "arm-none-eabi-gcc" "i686-w64-mingw32-gcc" "gdb"
 if [ $RUN_ARMCC -ne 0 ]; then
     check_tools "$ARMC5_CC" "$ARMC5_AR" "$ARMC6_CC" "$ARMC6_AR"
 fi