t3700: use test_grep helper for better diagnostics

t3700: use test_grep helper for better diagnostics

Replace 'grep' and '! grep' invocations with 'test_grep' and
'test_grep !'. This provides better debugging output if tests fail
in the future, as 'test_grep' will automatically print the
contents of the file when a check fails.

While at it, update any remaining instances of 'grep' to 'test_grep'
that were missed in the previous versions to ensure that the entire
file is consistent with modern project style.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>