tests: updated names for the dns tests

After run Sanitycheck script I found out that
some test cases have a same test case name
in the test result .xml file.
For boards mimxrt1050_evk, qemu_x86, sam_e70_xplained
in .xml files that cases were dublicated.
Problem happened only with cases net.dns.no_ipv6.init
and net.dns.init. Only that cases were dublicated.
To solve it, I decided to change test cases names for the dns tests.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>