Correct length check for DTLS records from old epochs.

DTLS records from previous epochs were incorrectly checked against the
current epoch transform's minimal content length, leading to the
rejection of entire datagrams. This commit fixed that and adapts two
test cases accordingly.

Internal reference: IOTSSL-1417
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index ae98ae9..22bc078 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -5126,8 +5126,8 @@
             0 \
             -c "replayed record" \
             -s "replayed record" \
-            -c "discarding invalid record" \
-            -s "discarding invalid record" \
+            -c "record from another epoch" \
+            -s "record from another epoch" \
             -S "resend" \
             -s "Extra-header:" \
             -c "HTTP/1.0 200 OK"
@@ -5139,8 +5139,8 @@
             0 \
             -c "replayed record" \
             -S "replayed record" \
-            -c "discarding invalid record" \
-            -s "discarding invalid record" \
+            -c "record from another epoch" \
+            -s "record from another epoch" \
             -c "resend" \
             -s "resend" \
             -s "Extra-header:" \
@@ -5201,8 +5201,6 @@
             0 \
             -c "record from another epoch" \
             -s "record from another epoch" \
-            -c "discarding invalid record" \
-            -s "discarding invalid record" \
             -s "Extra-header:" \
             -c "HTTP/1.0 200 OK"