selftests/bpf: Add default bpftool built by selftests to PATH

selftests/bpf: Add default bpftool built by selftests to PATH

For 'make run_tests':
selftests will build bpftool into tools/testing/selftests/bpf/tools/sbin/bpftool
by default.

==================
root@lkp-skl-d01 /opt/rootfs/v5.14-rc4# make -C tools/testing/selftests/bpf run_tests
make: Entering directory '/opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf'
  MKDIR    include
  MKDIR    libbpf
  MKDIR    bpftool
[...]
  GEN     /opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/tools/build/bpftool/profiler.skel.h
  CC      /opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/tools/build/bpftool/prog.o
  GEN     /opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/tools/build/bpftool/pid_iter.skel.h
  CC      /opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/tools/build/bpftool/pids.o
  LINK    /opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/tools/build/bpftool/bpftool
  INSTALL bpftool
  GEN      vmlinux.h
[...]
 # test_feature_dev_json (test_bpftool.TestBpftool) ... ERROR
 # test_feature_kernel (test_bpftool.TestBpftool) ... ERROR
 # test_feature_kernel_full (test_bpftool.TestBpftool) ... ERROR
 # test_feature_kernel_full_vs_not_full (test_bpftool.TestBpftool) ... ERROR
 # test_feature_macros (test_bpftool.TestBpftool) ... Error: bug: failed to retrieve CAP_BPF status: Invalid argument
 # ERROR
 #
 # ======================================================================
 # ERROR: test_feature_dev_json (test_bpftool.TestBpftool)
 # ----------------------------------------------------------------------
 # Traceback (most recent call last):
 #   File "/opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/test_bpftool.py", line 57, in wrapper
 #     return f(*args, iface, **kwargs)
 #   File "/opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/test_bpftool.py", line 82, in test_feature_dev_json
 #     res = bpftool_json(["feature", "probe", "dev", iface])
 #   File "/opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/test_bpftool.py", line 42, in bpftool_json
 #     res = _bpftool(args)
 #   File "/opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/test_bpftool.py", line 34, in _bpftool
 #     return subprocess.check_output(_args)
 #   File "/usr/lib/python3.7/subprocess.py", line 395, in check_output
 #     **kwargs).stdout
 #   File "/usr/lib/python3.7/subprocess.py", line 487, in run
 #     output=stdout, stderr=stderr)
 # subprocess.CalledProcessError: Command '['bpftool', '-j', 'feature', 'probe', 'dev', 'dummy0']' returned non-zero exit status 255.
 #
==================

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210820015556.23276-4-lizhijian@cn.fujitsu.com