tests/screenshots: Robust waits on shell state
Instead of relying solely on arbitrary sleep()s to ensure that the shell is in the state we want before taking a screenshot, watch for changes to PhoshShellStateFlags.
Related to #1051
Instead of relying solely on arbitrary sleep()s to ensure that the shell is in the state we want before taking a screenshot, watch for changes to PhoshShellStateFlags.
Related to #1051