selftests: net: fix available tunnels detection

The pmtu.sh test tries to detect the tunnel protocols available
in the running kernel and properly skip the unsupported cases.

In a few more complex setup, such detection is unsuccessful, as
the script currently ignores some intermediate error code at
setup time.

Before:
  # which: no nettest in (/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin)
  # TEST: vti6: PMTU exceptions (ESP-in-UDP)                            [FAIL]
  #   PMTU exception wasn't created after creating tunnel exceeding link layer MTU
  # ./pmtu.sh: line 931: kill: (7543) - No such process
  # ./pmtu.sh: line 931: kill: (7544) - No such process

After:
  #   xfrm4 not supported
  # TEST: vti4: PMTU exceptions                                         [SKIP]

Fixes: ece1278a

 ("selftests: net: add ESP-in-UDP PMTU test")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/cab10e75fda618e6fff8c595b632f47db58b9309.1706635101.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>