Automatically pass --as-module to the js binary when the test input
file has a .mjs extension. This allows us to write AST and bytecode
tests for module-only syntax like import and export statements.
Add a test runner for AST dump tests, modeled after test-js-bytecode.
Tests consist of JS input files and expected AST dump output, verified
by parsing with --parse-only --dump-ast --disable-ansi-colors.