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.
lethalman <notifications@github.com> writes:
>>          { buildInputs = [ makeWrapper];
>>            testScript = testScript';
>>            preferLocalBuild = true;
>> +          inherit testName;
>
> If testName is not needed in the environment, maybe it's better to use a passthru ?

I think it is nice to have that in the environment but currently don't
feel strongly about it. It enables testScript code to be shared among
tests and react differently.

-- 
Florian Friesdorf <flo@chaoflow.net>
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