Description:
------------
The mysqltest client has 'disable_parsing' and 'enable_parsing'
commands to disable or enable query parsing. These commands works same
as commenting out a section of a test and requires re-recording of the
test.

--disable_parsing
SELECT * FROM t1;
--enable_parsing

These commands are used inside few test files to comment out the test
cases and it might cause some confusion regarding why it's being used.

This Worklog

1. Renames [disable|enable]_parser commands to
   [disable|enable]_testcase. This setting is enabled by default. When
   disabled, mysqltest ignores everything until enable_testcase. These
   commands are useful for disabling a section causing an issue inside
   a test without having to disable the entire test.

2. Enforces bug number as the mandatory argument to disable_testcase
   command.

    --disable_testcase BUG#XXXX

   The bug number should be in 'BUG#XXXX' format where keyword 'BUG'
   is case-insensitive and 'XXXX' should contain only digits.

Change-Id: Ieba499206e2b73e666c17eaf21a68a755780aa6a