bpf: selftests: Specify CONFIG_DYNAMIC_FTRACE in the testing config

The tracing test and the recent kfunc call test require
CONFIG_DYNAMIC_FTRACE.  This patch adds it to the config file.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210403002921.3419721-1-kafai@fb.com