kunit: executor: Simplify string allocation handling

The alloc/copy code pattern is better consolidated to single kstrdup (and
kstrndup) calls instead. This gets rid of deprecated[1] strncpy() uses as
well. Replace one other strncpy() use with the more idiomatic strscpy().

Link: https://github.com/KSPP/linux/issues/90

 [1]
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>