Bug#37272728: main.time_zone relies on deprecated timezone name MET

Backport to 5.7.49.

Problem: Test used deprecated time zone name MET, and prints it in
result file. When OS changes the time zone name used when specifying
TZ=MET there is a result diff.

Solution: Switch from deprected MET to CET as the name for the time
zone.

Change-Id: I6c69c3ad62153a79c9337da4ecb4f5a13a248d56

Author: Dyre Tjeldvoll

mysql-test/r/timezone.result

@@ -1,7 +1,7 @@
 DROP TABLE IF EXISTS t1;
 show variables like "system_time_zone";
 Variable_name	Value
-system_time_zone	MET
+system_time_zone	CET
 select @a:=FROM_UNIXTIME(1);
 @a:=FROM_UNIXTIME(1)
 1970-01-01 01:00:01

mysql-test/t/timezone-master.opt

@@ -1 +1 @@
---timezone=MET
+--timezone=CET

mysql-test/t/timezone.test

@@ -1,7 +1,7 @@
 
 #
 # Test of SYSTEM time zone handling ( for my_system_gmt_sec()).
-# This script must be run with TZ=MET
+# This script must be run with TZ=CET
 
 -- require r/have_met_timezone.require
 disable_query_log;
@@ -14,7 +14,7 @@ DROP TABLE IF EXISTS t1;
 --enable_warnings
 
 # The following is because of daylight saving time
---replace_result MEST MET
+--replace_result MEST CET
 show variables like "system_time_zone";
 
 #