kunit: test: add test plan to KUnit TAP format

TAP 14 allows an optional test plan to be emitted before the start of
the start of testing[1]; this is valuable because it makes it possible
for a test harness to detect whether the number of tests run matches the
number of tests expected to be run, ensuring that no tests silently
failed.

Link[1]: https://github.com/isaacs/testanything.github.io/blob/tap14/tap-version-14-specification.md#the-plan
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/kunit/test_data/test_is_test_passed-all_passed.log

@@ -1,4 +1,5 @@
 TAP version 14
+1..2
 	# Subtest: sysctl_test
 	1..8
 	# sysctl_test_dointvec_null_tbl_data: sysctl_test_dointvec_null_tbl_data passed

tools/testing/kunit/test_data/test_is_test_passed-crash.log

@@ -1,6 +1,7 @@
 printk: console [tty0] enabled
 printk: console [mc-1] enabled
 TAP version 14
+1..2
 	# Subtest: sysctl_test
 	1..8
 	# sysctl_test_dointvec_null_tbl_data: sysctl_test_dointvec_null_tbl_data passed

tools/testing/kunit/test_data/test_is_test_passed-failure.log

@@ -1,4 +1,5 @@
 TAP version 14
+1..2
 	# Subtest: sysctl_test
 	1..8
 	# sysctl_test_dointvec_null_tbl_data: sysctl_test_dointvec_null_tbl_data passed