chainlint.pl: add parser to validate tests

chainlint.pl: add parser to validate tests

Continue fleshing out chainlint.pl by adding TestParser, a parser with
special knowledge about how Git tests should be written; for instance,
it knows that commands within a test body should be chained together
with `&&`. An upcoming parser which plucks test definitions from test
scripts will invoke TestParser for each test body it encounters.

Signed-off-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>