Add tests for builtin is

src/ascii_helpers.rs

src/builtins/exists.rs

src/builtins/is.rs

src/builtins/job_control.rs

src/builtins/man_pages.rs

src/builtins/mod.rs

src/builtins/random.rs

src/builtins/set.rs

src/builtins/test.rs

src/builtins/variables.rs

src/lib.rs

src/parser/arguments.rs

src/parser/assignments/actions.rs

src/parser/assignments/checker.rs

src/parser/assignments/keys.rs

src/parser/assignments/splitter.rs

src/parser/loops/for_grammar.rs

src/parser/pipelines/collector.rs

src/parser/pipelines/mod.rs

src/parser/quotes.rs