selftests: Update fib_tests to handle missing ping6

selftests: Update fib_tests to handle missing ping6

BugLink: https://bugs.launchpad.net/bugs/1848046
[ Upstream commit 0360894a05ed52be268e3c4d40b2df9d94975fa6 ]

Some distributions (e.g., debian buster) do not install ping6. Re-use
the hook in pmtu.sh to detect this and fallback to ping.

Fixes: a0e11da78f48 ("fib_tests: Add tests for metrics on routes")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>