[metif12]

fmt: make the tests read back what fmt wrote

Every test in this file compared an empty list against its expected output, and the
failure named a file argument rather than the thing that was wrong:

fmt: failed to open file "Now is the time for all good"

fmt is handed a closure to write through, and setup() built one that appended to a
&[]string pointing at a local in setup(). That local is gone by the time fmt runs,
so nothing was ever appended. Measured on current V:

closure appending to a captured `mut []string`   reads back empty
closure appending through a `&[]string`          reads back empty, and
                                                    crashes in array__push

A pointer to a heap struct does share, so the sink is a &Sink with a mut field
and the two closures capture that. The same trap in tail_test.v is #207.

fmt itself was never wrong here; it is handed the file, it reads it, and what came
back was being thrown away.

Tests

src/fmt is 1 of 1 and v fmt -verify src/fmt passes.

[JalonSolov]

Conflicts.