Add unit tests for parser and lexer

[Franklin-Qi]

Expand test coverage for the parser and lexer using existing patterns: test_parser! macro assertions for AST s-expressions and token-slice comparisons for the lexer.

Parser [Twelve new tests have been added (17 → 29)] :

• switch/case/default (including error cases)
• getline variants (<, |, |&)
• printf, control flow (next, nextfile, continue, return, exit)
• print redirection (>, >>, |, |&)
• regex matching (, !)
• function and indirect calls
• compound assignments (-=, *=, /=, ^=, %=)
• builtin variables, concurrent rules, namespace
• unary operators and division

Lexer [18 new tests have been added (24 → 42) ] :

• operators, compound assignments, and slash-assign disambiguation
• control-flow keywords and builtin variables (including ARGIND)
• typed regex, comments, directives (load, namespace)
• concurrent (non-GNU mode) and indirect calls (foo, ns::bar)
• regex literals, getline/print redirection, func (POSIX fallback)

Tests: cargo test -p lexer -p parser

Closes: #43

[Alonely0]

This is amazing, hands down. Tysm! We can merge right away :)