test: Rework test-suite input to avoid ulti-command strings

test: Rework test-suite input to avoid ulti-command strings

The original git test suite works by concatenating many commands into
a very long string (each separated by &&). This is painful to work
with since it prevents the editor from helping by parsing the shell
script, indenting, colorizing, etc.

Instead, we switch this back to something like the original notmuch
test suite, and add two new functions to test-lib.sh
(test_begin_subtest and test_expect_equal) to support these.

This also fixes the test suite to once again display the diff when a
test fails to generate the expected input.