(compiler+dub) flags to print file:line of the running unnitest

[Dejan Lekic]

This is not really a candidate for DIP so I write it here.

Many, many times in the past I needed to actually find unittest 
that is misbehaving and a simple feature would solve my problems 
- to be able to tell compiler to generate for each unittest 
output in form of `__FILE__ ~ ":" ~ __LINE__` so that I can 
figure out which unittest is taking too much time to execute.

Yes, I could use some third-party package but I do not want to 
add a new dependency to the project for something as trivial as 
this.

Currently when I need this I replace all my `unittest {` with

```d
unittest {
    tt("hunt");
```

Where tt is
```d
    void tt(string header, string fileName = __FILE__, int 
lineNumber = __LINE__)() {
       version (UT_HEADER) {
          import std.stdio: writeln;

          writeln("(", fileName, ":", lineNumber, ") ", header);
       }
    }
```

Once I find the culprit and fix or remove it, I then change back 
everything to unittests without calls to the `tt` template.

Can we _please_ have something like `dub test 
--print-ut-line-numbers` ?! Use whatever name you like instead of 
`--print-ut-line-numbers`.

Maybe there already is a way to do this, if there is please 
enlighten me!