Test case for sourceMaps set in swcrc.

[aptenodytes-forsteri]

Currently, the rules always set sourceMaps on the command line, overriding whatever is in swcrc. This can be confusing if we attempt to set sourceMaps in swcrc.

See #283

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Changes are visible to end-users: yes/no

• Searched for relevant documentation and updated as needed: yes/no
• Breaking change (forces users to change their own code or config): yes/no
• Suggested release notes appear below: yes/no

Test plan

• Covered by existing test cases
• New test cases added
• Manual testing; please provide instructions so we can reproduce:

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