Complete correctness for system variables

[spongedu]

In #7117 we add a framework to validate input when set system variables, but not all system variables are checked in that pr because there are too much of them. We should complete these checks step by step.

What to do:

1. Find out the specific limitation in MySQL system variable reference
2. Add related validation in method ValidateSetSystemVar in sessionctx/variable/varsutil.go
3. Add related tests in executor/set_test.go

Notes:

1. Details about MySQL system variables can be found here
2. executor,sessionctx: Add correctness for more system variables #7196 be be referd as an example.
3. System variables with ScopeNone scope need not be checked because they can't be changed dynamically.

[spongedu]

@shenli @zz-jason I think this issue can be marked as for-new-contributors ?

[zz-jason]

sure!

[spongedu]

#7196 deal with:

• sql_select_limit
• max_connect_errors
• table_definition_cache
• tmp_table_size
• connect_timeout

[laidahe]

Is there any other system variables need to add? @spongedu

[spongedu]

@laidahe you can take a look at https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html and #7196 , and find the remaining system variables in sessionctx/variable/sysvar.go