WL#11811: MTR: Rename and enhance [disable|enable]_parser test commands

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 WL

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

It is NOT a goal of this WL to

- check if a disabled section inside a test is still valid or not.

- check if there exist any bug for the disabled section of test cases

The above 2 tasks will be carried out in a separate WL or in a separate bug.

FR-1

disable_parsing and enable_parsing mysqltest commands will be renamed to disable_testcase and enable_testcase respectively

FR-2

All test cases between disable_testcase and enable_testcase commands will be disabled.

FR-3

Bug number is a mandatory argument to disable_testcase command i.e --disable_testcase BUG#XXXX

FR-4

Bug number argument should follow BUG#XXXX format

FR-4.1: "BUG" keyword is case-insensitive

FR-4.2: "XXXX" should contain only digits

FR-4.3: mysqltest client doesn't check if it's a valid bug number, it just checks for the format

1. Existing commands --disable_parsing and --enable_parsing are removed

2. Two new command --disable_testcase and --enable_testcase are introduced