Use more specific name in debug message for testing

While 'session hash' is currently unique, so suitable to prove that the
intended code path has been taken, it's a generic enough phrase that in the
future we might add other debug messages containing it in completely unrelated
code paths. In order to future-proof the accuracy of the test, let's use a
more specific string.
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 92eb62c..977903e 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -1210,8 +1210,8 @@
             -s "found extended master secret extension" \
             -s "server hello, adding extended master secret extension" \
             -c "found extended_master_secret extension" \
-            -c "session hash" \
-            -s "session hash"
+            -c "session hash for extended master secret" \
+            -s "session hash for extended master secret"
 
 run_test    "Extended Master Secret: client enabled, server disabled" \
             "$P_SRV debug_level=3 extended_ms=0" \
@@ -1221,8 +1221,8 @@
             -s "found extended master secret extension" \
             -S "server hello, adding extended master secret extension" \
             -C "found extended_master_secret extension" \
-            -C "session hash" \
-            -S "session hash"
+            -C "session hash for extended master secret" \
+            -S "session hash for extended master secret"
 
 run_test    "Extended Master Secret: client disabled, server enabled" \
             "$P_SRV debug_level=3 extended_ms=1" \
@@ -1232,8 +1232,8 @@
             -S "found extended master secret extension" \
             -S "server hello, adding extended master secret extension" \
             -C "found extended_master_secret extension" \
-            -C "session hash" \
-            -S "session hash"
+            -C "session hash for extended master secret" \
+            -S "session hash for extended master secret"
 
 requires_config_enabled MBEDTLS_SSL_PROTO_SSL3
 run_test    "Extended Master Secret: client SSLv3, server enabled" \
@@ -1244,8 +1244,8 @@
             -S "found extended master secret extension" \
             -S "server hello, adding extended master secret extension" \
             -C "found extended_master_secret extension" \
-            -C "session hash" \
-            -S "session hash"
+            -C "session hash for extended master secret" \
+            -S "session hash for extended master secret"
 
 requires_config_enabled MBEDTLS_SSL_PROTO_SSL3
 run_test    "Extended Master Secret: client enabled, server SSLv3" \
@@ -1256,8 +1256,8 @@
             -S "found extended master secret extension" \
             -S "server hello, adding extended master secret extension" \
             -C "found extended_master_secret extension" \
-            -C "session hash" \
-            -S "session hash"
+            -C "session hash for extended master secret" \
+            -S "session hash for extended master secret"
 
 # Tests for FALLBACK_SCSV