named nixos tests

Give nixos tests names:
```
/nix/store/wqvdm02yvvng2mdxvhk1l98149pkynix-vm-test-run-virtualenv-python2.7
```

This shows among others at the top of log files and in the target of the result links created by nix-build. Especially useful when building multiple tests with one nix-build run and examining log files later:
```
nix-build tests/installer.nix
...
firefox result*/log.html
```

Tests that do not define a name will show up as unnamed:
```
/nix/store/aanbj2x1c4giprj5nsrq84r2c5ka3sx0-vm-test-run-unnamed
```

If you like this I would name all existing nixos tests and include with this pull request.
Slightly more idiomatic is to have a function argument `name ? "unnamed"`.