Support additional tester macros

[morgo]

In addition to https://github.com/pingcap/mysql-tester/issues/24

The following macros in tests would be really helpful:

• --replace_column and --disable_result_log (these are used to patch out results from the output file. Used when testing RAND() and temporal values)
• --let and --eval - might be harder to implement but used by a lot of tests.
• --include

[mjonss]

With while also inc/dec is handy together with let/eval.

[mjonss]

Also --source is sometimes used, to repeat same queries within a single test.