Update staking unit tests

Since the ERC4626 deposit function has different caller and receiver
roles, let's test them here. This will reflect the flow for tBTC minting
and staking in one transaction, where an external contract will call the
stake function in the name of the staker.

core/test/Acre.test.ts

@@ -58,23 +58,28 @@ describe("Acre", () => {
         const expectedReceivedShares = amountToStake
 
         let tx: ContractTransactionResponse
+        let tbtcHolder: HardhatEthersSigner
+        let receiver: HardhatEthersSigner
 
         beforeEach(async () => {
+          tbtcHolder = staker1
+          receiver = staker2
+
           await tbtc
-            .connect(staker1)
+            .connect(tbtcHolder)
             .approve(await acre.getAddress(), amountToStake)
 
           tx = await acre
-            .connect(staker1)
-            .stake(amountToStake, staker1.address, referral)
+            .connect(tbtcHolder)
+            .stake(amountToStake, receiver.address, referral)
         })
 
         it("should emit Deposit event", () => {
           expect(tx).to.emit(acre, "Deposit").withArgs(
             // Caller.
-            staker1.address,
+            tbtcHolder.address,
             // Receiver.
-            staker1.address,
+            receiver.address,
             // Staked tokens.
             amountToStake,
             // Received shares.
@@ -91,15 +96,15 @@ describe("Acre", () => {
         it("should mint stBTC tokens", async () => {
           await expect(tx).to.changeTokenBalances(
             acre,
-            [staker1.address],
+            [receiver.address],
             [expectedReceivedShares],
           )
         })
 
         it("should transfer tBTC tokens", async () => {
           await expect(tx).to.changeTokenBalances(
             tbtc,
-            [staker1.address, acre],
+            [tbtcHolder.address, acre],
             [-amountToStake, amountToStake],
           )
         })