Write focus behavior unit test for form (#4100)

* Add comprehensive focus behavior test class

This test class verifies focus behavior using the @formtest annotation to
initialize the EDT and fire events through the entire stack. The tests cover:

- Focus state management (focusable, hasFocus, setFocus)
- Focus listeners (gained and lost events)
- Focus traversal (next/previous/left/right/up/down)
- Focus with containers and nested components
- Focus interaction with component state (enabled, visible)
- Focus with pointer/touch events
- Tab index and traversable properties
- Focus persistence across style changes
- Multiple focus listeners
- Focus cycles and form transitions

The implementation follows the existing test conventions:
- Extends UITestBase
- Uses @formtest for EDT initialization
- Avoids Mockito in favor of TestCodenameOneImplementation
- Limited to Java 8 syntax
- No reflection to access internal state
- Only modifies files in maven/core-unittests

* Fix focus behavior tests to match actual framework behavior

Updated three tests that had incorrect assumptions:

1. testDisabledComponentRetainsFocus (formerly testFocusLostWhenComponentDisabled):
   - Disabling a component doesn't automatically clear its focus
   - But disabled components cannot receive new focus via requestFocus()

2. testHiddenComponentCannotReceiveFocus (formerly testFocusWithHiddenComponent):
   - Making a component invisible doesn't automatically clear focus
   - But invisible components cannot receive new focus

3. testFocusTransferBetweenForms (formerly testFocusClearedOnFormChange):
   - Focus behavior during form transitions depends on form lifecycle
   - Tests that components on the new form can receive focus

These changes ensure tests verify actual framework behavior rather than
making incorrect assumptions about automatic focus management.

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Co-authored-by: Claude <[email protected]>

Author: shai-almog