selftests/bpf: replace CHECK calls with ASSERT macros in flow_dissector test

selftests/bpf: replace CHECK calls with ASSERT macros in flow_dissector test

The flow dissector test currently relies on generic CHECK macros to
perform tests. Update those to newer, more-specific ASSERT macros.

This update allows to get rid of the global duration variable, which was
needed by the CHECK macros

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241120-flow_dissector-v3-3-45b46494f937@bootlin.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>