[ios] Allow for iOS version slack for scenarios tests

[iskakaushik]

This is equivalent to having OS=latest which allows one
to pick the latest available OS version on the host system
rather than enforcing a strict match of 13.0

[dnfield]

I was under the impression we needed to pin the version because the golden testing we do on here for iOS can change from OS version to OS version. I'm a little worried that this will cause CI breakages when a toolchain rolls and it won't be obvious as to why. Any ideas about what we can do about that?

[dnfield]

/cc @LongCatIsLooong who did some work around updating this last time IIRC

[LongCatIsLooong]

I had to bump the simulator version for xcode 11 because there was no iPhone SE simulator by default in xcode 11, so I had to change it to iPhone 8. That would have been caught even if "OS" was not specified.

It seems "OS" was added in this PR because the golden images changed between OS versions?

[gaaclarke]

Yea like @dnfield said we have to be explicit here since the goldens won't match across operating system versions. You can install old versions of the simulator in Xcode. If you need help let me know.

[iskakaushik]

No, I was hoping that this would be a welcome change given the overhead of installing a specific version of the simulator. I am happy with leaving it as is if goldens require a specific version. Closing this PR.