Keep specs to unit tests and move lexer scenarios to PHPUnit

[chrisjenkinson]

LexerSpec had become an integration test. Most of its examples built real states and regex matchers, so they exercised AbstractState, RegexFinder, Cursor and Token as well as the lexer. This separates the two kinds of test:

• The end-to-end lexer scenarios move to a PHPUnit test, tests/Lexer/LexerTest.php, next to the existing NodeTraverserTest. They cover lookahead state switches, zero-length loops, idempotent tokenising, and token and error positions, including from a given start position.
• LexerSpec now uses state and token doubles and specifies only the lexer's own behaviour: the state stack, moving the cursor forward, where the cursor starts, detecting zero-length loops, and resetting between calls.
• SimpleNodeSpec's export example uses node doubles, so it checks only that a node exports its own attributes and children.

Value objects such as TokenPosition and MatchedText are still used directly in specs.