Re: Readd use of TAP subtests

Now that we have switched everything to done_testing(), the subtests
feature isn't that relevant anymore, but it might still be useful to get
better output when running with PROVE_FLAGS=--verbose. Compare before:

t/001_basic.pl ..
1..8
ok 1 - vacuumlo --help exit code 0
ok 2 - vacuumlo --help goes to stdout
ok 3 - vacuumlo --help nothing to stderr
ok 4 - vacuumlo --version exit code 0
ok 5 - vacuumlo --version goes to stdout
ok 6 - vacuumlo --version nothing to stderr
ok 7 - vacuumlo with invalid option nonzero exit code
ok 8 - vacuumlo with invalid option prints error message
ok
All tests successful.
Files=1, Tests=8, 0 wallclock secs ( 0.02 usr 0.00 sys + 0.08 cusr
0.05 csys = 0.15 CPU)
Result: PASS

After (with attached patch):

t/001_basic.pl ..
# Subtest: vacuumlo --help
ok 1 - exit code 0
ok 2 - goes to stdout
ok 3 - nothing to stderr
1..3
ok 1 - vacuumlo --help
# Subtest: vacuumlo --version
ok 1 - exit code 0
ok 2 - goes to stdout
ok 3 - nothing to stderr
1..3
ok 2 - vacuumlo --version
# Subtest: vacuumlo options handling
ok 1 - invalid option nonzero exit code
ok 2 - invalid option prints error message
1..2
ok 3 - vacuumlo options handling
1..3
ok
All tests successful.
Files=1, Tests=3, 0 wallclock secs ( 0.02 usr 0.01 sys + 0.11 cusr
0.07 csys = 0.21 CPU)
Result: PASS

I think for deeply nested tests and test routines defined in other
modules, this helps structure the test output more like the test source
code is laid out, so it makes following the tests and locating failing
test code easier.