ssl-opt.sh: Allow numerical constraints for tests

This commit adds functions requires_config_value_at_most()
and requires_config_value_at_least() which can be used to
only run tests when a numerical value from config.h
(e.g. MBEDTLS_SSL_IN_CONTENT_LEN) is within a certain range.
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index c12ca6a..bfcc634 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -156,6 +156,26 @@
     fi
 }
 
+requires_config_value_at_least() {
+    NAME="$1"
+    DEF_VAL=$( grep ".*#define.*MBEDTLS_SSL_DTLS_MAX_BUFFERING" ../include/mbedtls/config.h |
+               sed 's/^.*\s\([0-9]*\)$/\1/' )
+    VAL=$( ../scripts/config.pl get $NAME || echo "$DEF_VAL" )
+    if [ "$VAL" -lt "$2" ]; then
+       SKIP_NEXT="YES"
+    fi
+}
+
+requires_config_value_at_most() {
+    NAME="$1"
+    DEF_VAL=$( grep ".*#define.*MBEDTLS_SSL_DTLS_MAX_BUFFERING" ../include/mbedtls/config.h |
+               sed 's/^.*\s\([0-9]*\)$/\1/' )
+    VAL=$( ../scripts/config.pl get $NAME || echo "$DEF_VAL" )
+    if [ "$VAL" -gt "$2" ]; then
+       SKIP_NEXT="YES"
+    fi
+}
+
 # skip next test if OpenSSL doesn't support FALLBACK_SCSV
 requires_openssl_with_fallback_scsv() {
     if [ -z "${OPENSSL_HAS_FBSCSV:-}" ]; then