Add PHPUnit test suite

Problem/Motivation

No tests

Proposed resolution

Add following tests:

• tests/src/Unit/TinySliderGlobalTest.php — 20 tests covering defaultSettings()/formatSettings()/isValidJson(): the float/int cast fixes, the itemsMobile/itemsDesktop guard logic, the navContainer/uniqueFieldID ordering fix, configJson round-tripping — essentially every bug this session found in that class, now locked in.
• tests/src/Kernel/TinySliderFieldFormatterTest.php — 4 tests, including direct regressions for #3479131 (image linked to file, and to content — both used to crash).
• tests/src/Kernel/Views/TinySliderStyleTest.php — 4 tests covering Views option defaults matching TinySliderGlobal and slideBy validation.
• config/schema/tiny_slider.schema.yml — new, required for tests to run at all.