otest-query-ios now launches via iPhoneSimulator.platform/usr/bin/sim

This fixes the bug that started in 2a7abf856 where otest-query would
fail with --

```
failed: Failed to query the list of test cases in the test bundle: Error while parsing JSON: The data has been corrupted.:
```

The problem turns out to be how we set CFFIXED_USER_HOME - I don't quite
understand why, but I've narrowed it down to that.

In that commmit, we began setting CFFIXED_USER_HOME
iOS app tests don't need CFFIXED_USER_HOME, DYLD_ROOT_PATH, ...

... or IPHONE_SIMULATOR_ROOT.  Since we launch the tests in the iOS
simulator, it will take care of setting these variables.

Tested by removing these lines, running tests, and verifying that the
vars are still set.
run iOS logic tests via iPhoneSimulator.platform/usr/bin/sim like Xcode

Xcode actually doesn't launch otest/xctest directly - it hops through
this 'sim' program first.

The nice thing about 'sim' is that it populates all the IPHONE_*
environment variables for us, which lets us drop a bunch of code.
adding CreateTaskForSimulatorExecutable() for launching iOS binaries

This makes it easier to use the 'iPhoneSimulator.platform/usr/bin/sim'
launcher.