Disable DTLS proxy tests for MEMORY_BUFFER_ALLOC test

diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 77f5188..2ff0ebf 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -938,7 +938,8 @@
     make test
 
     msg "test: ssl-opt.sh, MBEDTLS_MEMORY_BUFFER_ALLOC_C"
-    if_build_succeeded tests/ssl-opt.sh
+    # MBEDTLS_MEMORY_BUFFER_ALLOC is slow. Skip tests that tend to time out.
+    if_build_succeeded tests/ssl-opt.sh -e '^DTLS proxy'
 }
 
 component_test_no_max_fragment_length () {