selftests/bpf: NUL-terminate bpftool command output

run_command() writes a bpftool command's output into a caller-supplied
buffer, and every caller treats that buffer as a C string.

However fread() reports a byte count and doesn't terminate the
string. The helper does not terminate either, so callers have to zero
the buffer first. prog_tests/bpftool_metadata.c does not, for example.

Read one byte less and terminate in the helper.

Fixes: f21fae577446 ("selftests/bpf: Add a few helpers for bpftool testing")
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260828215207.3105313-5-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>