selftests/net: convert ioam6.sh to run it in unique namespace

Here is the test result after conversion.

]# ./ioam6.sh

--------------------------------------------------------------------------
OUTPUT tests
--------------------------------------------------------------------------
TEST: Unknown IOAM namespace (inline mode)                          [ OK ]
TEST: Unknown IOAM namespace (encap mode)                           [ OK ]
TEST: Missing trace room (inline mode)                              [ OK ]
TEST: Missing trace room (encap mode)                               [ OK ]
TEST: Trace type with bit 0 only (inline mode)                      [ OK ]
...
TEST: Full supported trace (encap mode)                             [ OK ]

--------------------------------------------------------------------------
GLOBAL tests
--------------------------------------------------------------------------
TEST: Forward - Full supported trace (inline mode)                  [ OK ]
TEST: Forward - Full supported trace (encap mode)                   [ OK ]

- Tests passed: 88
- Tests failed: 0
Acked-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>