kunit: Report test parameter results as (K)TAP subtests

Currently, the results for individial parameters in a parameterised test
are simply output as (K)TAP diagnostic lines.

As kunit_tool now supports nested subtests, report each parameter as its
own subtest.

For example, here's what the output now looks like:
	# Subtest: inode_test_xtimestamp_decoding
	ok 1 - 1901-12-13 Lower bound of 32bit < 0 timestamp, no extra bits
	ok 2 - 1969-12-31 Upper bound of 32bit < 0 timestamp, no extra bits
	ok 3 - 1970-01-01 Lower bound of 32bit >=0 timestamp, no extra bits
	ok 4 - 2038-01-19 Upper bound of 32bit >=0 timestamp, no extra bits
	ok 5 - 2038-01-19 Lower bound of 32bit <0 timestamp, lo extra sec bit on
	ok 6 - 2106-02-07 Upper bound of 32bit <0 timestamp, lo extra sec bit on
	ok 7 - 2106-02-07 Lower bound of 32bit >=0 timestamp, lo extra sec bit on
	ok 8 - 2174-02-25 Upper bound of 32bit >=0 timestamp, lo extra sec bit on
	ok 9 - 2174-02-25 Lower bound of 32bit <0 timestamp, hi extra sec bit on
	ok 10 - 2242-03-16 Upper bound of 32bit <0 timestamp, hi extra sec bit on
	ok 11 - 2242-03-16 Lower bound of 32bit >=0 timestamp, hi extra sec bit on
	ok 12 - 2310-04-04 Upper bound of 32bit >=0 timestamp, hi extra sec bit on
	ok 13 - 2310-04-04 Upper bound of 32bit>=0 timestamp, hi extra sec bit 1. 1 ns
	ok 14 - 2378-04-22 Lower bound of 32bit>= timestamp. Extra sec bits 1. Max ns
	ok 15 - 2378-04-22 Lower bound of 32bit >=0 timestamp. All extra sec bits on
	ok 16 - 2446-05-10 Upper bound of 32bit >=0 timestamp. All extra sec bits on
	# inode_test_xtimestamp_decoding: pass:16 fail:0 skip:0 total:16
	ok 1 - inode_test_xtimestamp_decoding

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>