Started specifying the OS version for running the tests.

[gaaclarke]

relevant issue: flutter/flutter#46120

[liyuqian]

I wonder what's the motivation of this change? If there's an issue, maybe a link to it would be helpful.

[gaaclarke]

I wonder what's the motivation of this change? If there's an issue, maybe a link to it would be helpful.

Added. We should always specify the OS version the golden tests are meant to be run on since changes in OS's can break golden tests.

[liyuqian]

LGTM

[dnfield]

LGTM - we'll need to update this periodically though, e.g. when we want to update https://github.com/flutter/infra/blob/master/config/main.star#L254 and it no longer includes this version.

[gaaclarke]

I downgraded the OS version to 12.2 since @dnfield told me we are running Xcode 10.2 on the testing machines which means it should have the simulator for 12.2 already installed (https://developer.apple.com/documentation/ios_ipados_release_notes/ios_12_2_release_notes?language=objc).

[gaaclarke]

LGTM - we'll need to update this periodically though, e.g. when we want to update https://github.com/flutter/infra/blob/master/config/main.star#L254 and it no longer includes this version.

Heads up, we are already going to run into troubles because we are using iPhone SE. On Xcode 11+ that is no longer a standard simulator target. Higher versions of Xcode can test on lower versions, you just have to go through the effort to setup and install the simulators.

[gaaclarke]

Verified the change in luci build: https://ci.chromium.org/swarming/task/48ec2fcae723a110?server=chromium-swarm.appspot.com