[Bug d/103728] Allow to run unittests before main()

["ibuclaw at gdcproject dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103728

--- Comment #1 from Iain Buclaw <ibuclaw at gdcproject dot org> ---
Not sure if there'd be much help from the compiler on this front, as it's a
runtime library feature that unittests are executed in the first place - the
compiler simply generates functions in the proper place for the library to find
them.

See
https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=libphobos/libdruntime/rt/dmain2.d;h=b363e3fea4fc55d9f6941030dc808e5129bd38c2;hb=HEAD#l491

The old behaviour was of `runAll()` was:

    if (rt_init() && runModuleUnitTests())
        tryExec({ result = mainFunc(args); });

> someone suggested to use this:
> version (unittest) extern(C) __gshared string[] rt_options = [ "testmode=run-
> main" ];
> It works well with gdc-12, but it is very cryptic and still hard to add in all
> our binaries (we have dozens).
A second way of doing the would be to run the program with the command-line
option: --DRT-testmode=run-main

The options itself gets filtered from the `string[] args` parameter of D main,
so it won't affect tests where arguments are given to binaries (unless the
--DRT option appears after `--`).