SHA-1 deprecation: allow it in key exchange

By default, keep allowing SHA-1 in key exchange signatures. Disabling
it causes compatibility issues, especially with clients that use
TLS1.2 but don't send the signature_algorithms extension.

SHA-1 is forbidden in certificates by default, since it's vulnerable
to offline collision-based attacks.
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index f498732..8170413 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -2684,12 +2684,19 @@
 # Test for ClientHello without extensions
 
 requires_gnutls
-run_test    "ClientHello without extensions" \
+run_test    "ClientHello without extensions, SHA-1 allowed" \
             "$P_SRV debug_level=3" \
             "$G_CLI --priority=NORMAL:%NO_EXTENSIONS:%DISABLE_SAFE_RENEGOTIATION" \
             0 \
             -s "dumping 'client hello extensions' (0 bytes)"
 
+requires_gnutls
+run_test    "ClientHello without extensions, SHA-1 forbidden in certificates on server" \
+            "$P_SRV debug_level=3 key_file=data_files/server2.key crt_file=data_files/server2.crt allow_sha1=0" \
+            "$G_CLI --priority=NORMAL:%NO_EXTENSIONS:%DISABLE_SAFE_RENEGOTIATION" \
+            0 \
+            -s "dumping 'client hello extensions' (0 bytes)"
+
 # Tests for mbedtls_ssl_get_bytes_avail()
 
 run_test    "mbedtls_ssl_get_bytes_avail: no extra data" \