Making the test for MDEV-17854 independent from the host time zone

mysql-test/main/func_time.result

@@ -6298,9 +6298,11 @@ TIME('- 01:00:00')	TIME('- 1 01:00:00')
 #
 # MDEV-17854 Assertion `decimals <= 6' failed in my_time_fraction_remainder on SELECT with NULLIF and FROM_UNIXTIME on incorrect time
 #
+SET time_zone='+00:00';
 SELECT NULLIF(FROM_UNIXTIME('foo'), '2012-12-12 21:10:14');
 NULLIF(FROM_UNIXTIME('foo'), '2012-12-12 21:10:14')
-1970-01-01 04:00:00
+1970-01-01 00:00:00
 Warnings:
 Warning	1292	Truncated incorrect DECIMAL value: 'foo'
 Warning	1292	Truncated incorrect DECIMAL value: 'foo'
+SET time_zone=DEFAULT;

mysql-test/main/func_time.test

@@ -3129,4 +3129,6 @@ SELECT TIME('- 01:00:00'), TIME('- 1 01:00:00');
 --echo # MDEV-17854 Assertion `decimals <= 6' failed in my_time_fraction_remainder on SELECT with NULLIF and FROM_UNIXTIME on incorrect time
 --echo #
 
+SET time_zone='+00:00';
 SELECT NULLIF(FROM_UNIXTIME('foo'), '2012-12-12 21:10:14');
+SET time_zone=DEFAULT;