scenario file dependencies

[zeroXbrock]

Is your feature request related to a problem? Please describe.

we could compose scenarios separately to make them more readable and reduce duplication.

Describe the solution you'd like

add a requires field to the top-level scenario spec.

requires=["weth.toml", "uniV2.toml"]

[spam]
# ...

Then when parsing a TestConfig add the [create] and [setup] steps from the requires files to the given scenario config, in order and before any steps in the given config.