Allow for separate tests config for wp-env

[talldan]

Is your feature request related to a problem? Please describe.
#20280 enabled using wp-env for e2e tests in travis, but unearthed a few minor issues.

One of which is that SCRIPT_DEBUG is historically set to false for e2e tests and true for a dev environment. Currently wp-env only supports a single config for both environments. The workaround for travis was to output a .wp-env.override file:
https://github.com/WordPress/gutenberg/blob/master/.travis.yml#L56

This isn't very maintainable because it overrides all settings.

Describe the solution you'd like
A separate configuration for tests.

Describe alternatives you've considered
Command line options for overriding individual settings, e.g.

wp-env start --override-config SCRIPT_DEBUG=false