Added missing tests for Standalone fusebox shell

[chicio]

Summary:

This PR adds the missing test coverage for the standalone Fusebox debugger shell in StandaloneFuseboxShell-test.
First, I added some missing expectation for the success case (enableStandaloneFuseboxShell=true and no errors) already present:

• the assertion that unstable_prepareFuseboxShell is called just one time during middleware initialization
• the assertion that launchDebuggerAppWindow is never called when the standalone shell is available

Then, I added the missing test for the case when unstable_prepareFuseboxShell returns one of the error code like eg. platform_not_supported (as reported also in the TODO that I removed).
In this way now the test is verifying the full behaviour for the new standalone mode for debugger/dev tools.

Changelog:

[GENERAL] [CHANGED] - Added missing tests for Standalone fusebox shell

Test Plan:

• Ran StandaloneFuseboxShell-test and verify all tests cases passed
• Ran the full React Native test suite to ensure all tests pass.
• Verified test correctness by intentionally breaking production code:
  • removing the calls to the DefaultBrowserLauncher in openDebuggerMiddleware to verify that the tests were correctly failing