Remove clj-kondo warning that := is an unused value

[vedang]

clj-kondo does not understand that := is special in rcf/tests. This commit adds a clj-kondo.exports folder as explained in their documentation. Users can use this to import the config in their projects and remove the warning.

References:

1. Exporting and importing clj-kondo configuration: https://github.com/clj-kondo/clj-kondo/blob/74be7d29696c08e188d10c64b045403b21302a0b/doc/config.md?plain=1#L524
2. An explanation of the :config-in-call option: https://github.com/clj-kondo/clj-kondo/blob/74be7d29696c08e188d10c64b045403b21302a0b/doc/config.md#config-in-call
3. An explanation of the :unused-value warning that we see: https://github.com/clj-kondo/clj-kondo/blob/74be7d29696c08e188d10c64b045403b21302a0b/doc/linters.md#unused-value

[dustingetz]

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