How to convert an address to a contract in LIGO testing framework ?

In the context of testing framework, if you want to convert an address to a contract, you need to convert address to typed_address using Test.cast_address.

Then convert typed_address to contract using Test.to_contract. For example:

let test =
  let addr = ("tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx" : address) in
  let taddr : (unit, unit) typed_address = Test.cast_address addr in
  let contract : (unit) contract = Test.to_contract taddr in
  contract

const test = do {
  const addr = ("tz1KqTpEZ7Yob7QbPE4Hy4Wo8fHG8LhKxZSx" as address);
  const taddr : typed_address<unit,unit> = Test.cast_address(addr);
  const contract : contract<unit> = Test.to_contract(taddr);
  return contract;
};

Check out the reference of the Test framework for exact signature of the functions here.