bats: add usage example to longDescription

The `bats` package provides a `withLibraries` function to easily include helper library such as `bats-assert` and `bats-support`.
However, this capability is not immediately visible to users browsing packages via `Nixpkgs Package Search`.

This commit adds a `longDescription` to document this feature, providing a code example on how to compose `bats` with its libraries.

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