test: require test_begin_subtest before test_expect_code

test: require test_begin_subtest before test_expect_code

Unify the subtests by requiring test_begin_subtest before
test_expect_code. (Similar change for test_expect_success has already
been done.)

This increases clarity in the test scripts by having a separate line
for the start of the subtest with the heading, and makes it possible
to simplify the test infrastructure by making all subtests similar.