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.

tests/scripts/test_zeroize.gdb

@@ -0,0 +1,25 @@
+set confirm off
+file ./programs/test/zeroize
+break zeroize.c:90
+
+set args ./programs/test/zeroize.c
+run
+
+set $i = 0
+set $len = sizeof(buf)
+set $buf = buf
+
+if exit_code != 0
+    echo The program did not terminate correctly\n
+    quit 1
+end
+
+while $i < $len
+    if $buf[$i++] != 0
+        echo The buffer at was not zeroized\n
+        quit 1
+    end
+end
+
+echo The buffer was correctly zeroized\n
+quit 0