test(testutil): add keeper reference mocks in fungible test keeper

[lumtis]

Description

Replace empty keeper instances with mocks in fungible keepers for isolated unit testing.

The mocking system uses mockery utility: https://vektra.github.io/mockery/latest/

The mocks are generated by running the generation commands in testutil/keeper/mocks.go:

//go:generate mockery --name FungibleAccountKeeper --filename account.go --case underscore --output ./mocks/fungible
type FungibleAccountKeeper interface {
	fungibletypes.AccountKeeper
}

//go:generate mockery --name FungibleBankKeeper --filename fungible.go --case underscore --output ./mocks/fungible
type FungibleBankKeeper interface {
	fungibletypes.BankKeeper
}

//go:generate mockery --name FungibleObserverKeeper --filename observer.go --case underscore --output ./mocks/fungible
type FungibleObserverKeeper interface {
	fungibletypes.ObserverKeeper
}

The generated structures are located in ..../mocks/[modulename]/[mockedmodule].go

The generated structures can be initialized with the New... methods

Dependent on #900

Closes: #901

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Include instructions and any relevant details so others can reproduce.

• Tested CCTX in localnet
• Tested in development environment
• Go unit tests
• Go integration tests
• Tested via GitHub Actions

Checklist:

• I have added unit tests that prove my fix feature works