mysqltest: source inside a loop

The current implementation of caching in mysqltest (q_lines, parser) breaks if the source command is executed conditionally in a loop.

There is a "fix" for that — commit fe3ea31d93 — where the source command is executed on the first iteration of a loop even if it should've been skipped, just to read the file into a cache.

This is, of course, wrong, because it fails if the file name is not available on the first iteration, or if t changes between iterations:

let $a=1;

while ($a < 3) {

  if ($b) {

    source $b;

  }

  let $b=1st.test;

  inc $a;

}

let $a=1;

let $b=1st.test;

while ($a < 3) {

  if ($b) {

    source $b;

  }

  let $b=2nd.test;

  inc $a;

}