make tests pass even when some iOS sim SDKs are missing.
make tests pass even when some iOS sim SDKs are missing.

Summary:
This makes tests pass even if you don't have the 5.0, 5.1, 6.0, 6.1 SDKs
installed.

This is pretty ugly.  In an ideal world, we'd have plumbing so that we
could pass the available SDKs from above (in our tests).

Test Plan:
In...

```
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs`
```

... I moved everything but the 6.1 SDK aside and ran tests.  They
passed.

soundcloud/xctool,adamjernst/xctool,n054/xctool,githubdelegate/xctool,kamoljan/xctool,n054/xctool,lpios/xctool,ACChe/xctool,facebook/xctool,soundcloud/xctool,kcoons/xctool,n054/xctool,blighli/xctool,LegNeato/xctool,blighli/xctool,dvor/xctool,jspahrsummers/xctool,duk42111/xctool,kamoljan/xctool,1yvT0s/xctool,ACChe/xctool,PSPDFKit-labs/xctool,adamjernst/xctool,facebook/xctool,duk42111/xctool,trello/xctool,yaoxiaoyong/xctool,LegNeato/xctool,agreco/xctool,n054/xctool,Richardlihui/xctool,theiren46/xctool,ACChe/xctool,kernandreas/xctool,kernandreas/xctool,dvor/xctool,lqi/xctool,1yvT0s/xctool,kernandreas/xctool,ekingo/xctool,agreco/xctool,PSPDFKit-labs/xctool,ekingo/xctool,jspahrsummers/xctool,kcoons/xctool,SuPair/xctool,modulexcite/xctool,leopardpan/xctool,bgertzfield/xctool,FiftyThree/xctool,soundcloud/xctool,bhamiltoncx/xctool,adamjernst/xctool,dvor/xctool,bhamiltoncx/xctool,Monits/xctool,n054/xctool,Richardlihui/xctool,agreco/xctool,kcoons/xctool,lqi/xctool,githubdelegate/xctool,SuPair/xctool,githubdelegate/xctool,bhamiltoncx/xctool,facebook/xctool,lpios/xctool,modulexcite/xctool,blighli/xctool,theiren46/xctool,yaoxiaoyong/xctool,duk42111/xctool,lpios/xctool,ACChe/xctool,LegNeato/xctool,lqi/xctool,bgertzfield/xctool,leopardpan/xctool,kamoljan/xctool,jspahrsummers/xctool,githubdelegate/xctool,ekingo/xctool,trello/xctool,leopardpan/xctool,lpios/xctool,LegNeato/xctool,FiftyThree/xctool,bgertzfield/xctool,AutoScout24/xctool,0xced/xctool,AutoScout24/xctool,soundcloud/xctool,dvor/xctool,Richardlihui/xctool,Monits/xctool,lpios/xctool,0xced/xctool,ACChe/xctool,trello/xctool,leopardpan/xctool,SuPair/xctool,yaoxiaoyong/xctool,yaoxiaoyong/xctool,agreco/xctool,AutoScout24/xctool,1yvT0s/xctool,facebook/xctool,adamjernst/xctool,facebook/xctool,kamoljan/xctool,theiren46/xctool,Richardlihui/xctool,bhamiltoncx/xctool,lpios/xctool,bgertzfield/xctool,AutoScout24/xctool,bhamiltoncx/xctool,bgertzfield/xctool,kernandreas/xctool,soundcloud/xctool,bgertzfield/xctool,modulexcite/xctool,FiftyThree/xctool,SuPair/xctool,FiftyThree/xctool,Monits/xctool,PSPDFKit-labs/xctool,facebook/xctool,bhamiltoncx/xctool,duk42111/xctool,1yvT0s/xctool,ACChe/xctool,jspahrsummers/xctool,PSPDFKit-labs/xctool,blighli/xctool,PSPDFKit-labs/xctool,modulexcite/xctool,soundcloud/xctool,trello/xctool,Monits/xctool,n054/xctool,theiren46/xctool,ekingo/xctool