Fix support for running multiple web flows

[jkronborg]

Proposed changes

Support running multiple web flows in one test command. This used to work when using the --device=chromium option, but broke in version 1.39.0 (commit).

Testing

# Workspace containing web flows
maestro test path/to/web-workspace

# Multiple web flows in a directory
maestro test some/directory/web-*.yaml

# Single web flow
maestro test some/directory/web-flow1.yaml

# Workspace containing iOS flows
maestro test path/to/ios-workspace

# Multiple iOS flows in a directory
maestro test some/directory/ios-*.yaml

# Single web flow
maestro test some/directory/ios-flow1.yaml

# A mix of web flows and iOS flows (fails)
maestro test path/to/mixed-workspace

I'm not really familiar with the codebase, so I don't know if this change has any unintended consequences not caught by the tests above.

Automated test coverage of this could be added in #2195.

Issues fixed

#2269