selftests/bpf: make fexit_stress test run in serial mode

fexit_stress is attaching maximum allowed amount of fexit programs to
bpf_fentry_test1 kernel function, which is used by a bunch of other
parallel tests, thus pretty frequently interfering with their execution.

Given the test assumes nothing else is attaching to bpf_fentry_test1,
mark it serial.

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220511232012.609370-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>