test(ramps): fixes btc native token test (#27601)

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## **Description**

The ramps ducks test incorrectly tested for BTC as a native asset. This
scenario is not possible and should not have been tested. Bitcoin
doesn't use the NetworkController, so BTC cannot be passed to the
`getCurrentChainIdMock`.


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## **Related issues**

Fixes:

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ui/ducks/ramps/ramps.test.ts

@@ -205,7 +205,7 @@ describe('rampsSlice', () => {
     });
 
     it('should return true when Bitcoin is buyable and current chain is Bitcoin', () => {
-      getCurrentChainIdMock.mockReturnValue(MultichainNetworks.BITCOIN);
+      getCurrentChainIdMock.mockReturnValue(CHAIN_IDS.MAINNET);
       getMultichainIsBitcoinMock.mockReturnValue(true);
       const mockBuyableChains = [
         { chainId: MultichainNetworks.BITCOIN, active: true },
@@ -219,7 +219,7 @@ describe('rampsSlice', () => {
     });
 
     it('should return false when Bitcoin is not buyable and current chain is Bitcoin', () => {
-      getCurrentChainIdMock.mockReturnValue(MultichainNetworks.BITCOIN);
+      getCurrentChainIdMock.mockReturnValue(CHAIN_IDS.MAINNET);
       getMultichainIsBitcoinMock.mockReturnValue(true);
       const mockBuyableChains = [
         { chainId: MultichainNetworks.BITCOIN, active: false },