Unit tests via DUB

[alexanderzhirov]

I don't quite understand why compiling unit tests using DUB 
doesn't work.

JSON:
```json
{
   "authors": [
     "alexander"
   ],
   "description": "Array Slicing",
   "license": "proprietary",
   "name": "array_slicing",
   "targetName": "program",
   "targetPath": "bin"
}
```
Projects structure:
```sh
├── bin
├── dub.json
└── source
     └── app.d
```
Errors:
```sh
No source files found in configuration 'library'. Falling back to 
"dub -b unittest".
Performing "unittest" build using /usr/bin/dmd for x86_64.
array_slicing ~master: building configuration "application"...
Linking...
/usr/bin/ld: 
/usr/lib64/gcc/x86_64-solus-linux/11/../../../../lib64/crt1.o: в 
функции «_start»:
/home/build/YPKG/root/glibc/build/glibc.git/csu/../sysdeps/x86_64/start.S:110: неопределённая ссылка на «main»
collect2: ошибка: выполнение ld завершилось с кодом возврата 1
Error: linker exited with status 1
/usr/bin/dmd failed with exit code 1.
```