test-writing-guidelines: Fixed several small typos

Signed-off-by: Jiri Jaburek <jjaburek@redhat.com>
test-writing-guidelines: more typo/syntax fixes

+ one TODO added

Signed-off-by: Jiri Jaburek <jjaburek@redhat.com>
test-writing-guidelines: More typo fixes

Signed-off-by: Jiri Jaburek <jjaburek@redhat.com>
test-writing-guidelines: added info about tst_get_unused_pid()

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
test-writing-guidelines: Clarify the TODO

This (hopefully) clarifies the TODO added by Jiri Jaburek.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>