test(): clean up bulkMembers tests and fix claim test isolation issue

- Simplified DirectPayments.bulkMembers.test.ts by removing unnecessary
  custom validator creation and using existing fixture mocks with proper
  withArgs configuration

- Simplified UBIPool.bulkMembers.test.ts by removing custom validator
  creation in validation tests and using fixture mocks instead

- Changed UBIPool uniqueness validation test to use signers[5-7] instead
  of signers[2-4] to avoid mock conflicts with other tests

- Fixed DirectPayments.claim.test.ts "non member should not be able to
  get rewards" test by explicitly setting uniquenessValidator to AddressZero
  to prevent "Mock on the method is not initialized" error when running
  all tests together

The claim test was failing when run with other tests because it inherited
uniqueness validator mock state without proper initialization for the test's
specific address. Setting uniquenessValidator to zero address ensures proper
test isolation and bypasses uniqueness validation, which is appropriate
since this test only validates member validation logic.

Author: EmekaManuel

packages/contracts/test/DirectPayments/DirectPayments.claim.test.ts

@@ -175,7 +175,7 @@ describe('DirectPaymentsPool Claim', () => {
 
       newMembersValidator.mock['isMemberValid'].returns(false);
 
-      await pool.setPoolSettings({ ...poolSettings, membersValidator: newMembersValidator.address }, 0);
+      await pool.setPoolSettings({ ...poolSettings, membersValidator: newMembersValidator.address, uniquenessValidator: ethers.constants.AddressZero }, 0);
       await expect(pool['claim(uint256)'](nftSampleId)).not.reverted;
       const contributer = nftSample.events[0].contributers[0];
       const initialBalance = await gdframework.GoodDollar.balanceOf(contributer);