Adding a test for "MDEV-6973 XOR aggregates argument collations".

The bug itself was earlier fixed by this patch:

commit 3709c7fc
MDEV-8222 "string_field LIKE int_const" returns a wrong result in case of UCS2
MDEV-8257 Erroneous "Impossible where" when mixing decimal comparison and LIKE

mysql-test/r/ctype_uca.result

@@ -13573,3 +13573,15 @@ DROP TABLE t1;
 #
 # End of MariaDB-10.0 tests
 #
+#
+# Start of MariaDB-10.1 tests
+#
+#
+# MDEV-6973 XOR aggregates argument collations
+#
+SELECT '10' COLLATE utf8_general_ci XOR '20' COLLATE utf8_unicode_ci;
+'10' COLLATE utf8_general_ci XOR '20' COLLATE utf8_unicode_ci
+0
+#
+# End of MariaDB-10.1 tests
+#

mysql-test/t/ctype_uca.test

@@ -620,3 +620,17 @@ DROP TABLE t1;
 --echo #
 --echo # End of MariaDB-10.0 tests
 --echo #
+
+--echo #
+--echo # Start of MariaDB-10.1 tests
+--echo #
+
+--echo #
+--echo # MDEV-6973 XOR aggregates argument collations
+--echo #
+SELECT '10' COLLATE utf8_general_ci XOR '20' COLLATE utf8_unicode_ci;
+
+
+--echo #
+--echo # End of MariaDB-10.1 tests
+--echo #