test: add comprehensive tests for Tumblr readers (#17)

TumblrReader tests (21):
- load() error handling
- loadFromUrl() with options and callbacks
- Navigation (next, prev, goTo)
- History management
- Cache integration
- getNavigation, hasNavigation
- getCurrentPost, getBlogName
- destroy cleanup

TumblrPlaylistReader tests (21):
- load() error handling
- loadFromPlaylist() with URL extraction
- Navigation (next, prev, goTo)
- ToC generation and navigation
- hasNavigation states
- prefetchAll with progress
- clearCache and ToC updates
- exportAsEpub
- destroy cleanup

Increases test count from 149 to 191 (+42 tests)

Author: gvonnessi