[testing] add skipUnlessHasBinaries decorator

Some tests are currently allowed to fail with @mayFail because some binaries may be missing.
The problem is that we cannot differenciate between a test failing because some tooling is missing,
or tests legitimately failing.

This diff creates a new decorator that will skip a test if some binaries are not
present, therefore, if the binaries are there and there is any issue, CI will fail.

It will generate a message similar to:
```
docker run -ti \
                    --privileged \
                    --network=host \
                    --pid=host \
                    -v $(pwd):/bcc \
                    -v /sys/kernel/debug:/sys/kernel/debug:rw \
                    -v /lib/modules:/lib/modules:ro \
                    -v /usr/src:/usr/src:ro \
                    -e CTEST_OUTPUT_ON_FAILURE=1 \
                    u34 \
                    /bin/bash -c \
                    '/bcc/build/tests/wrapper.sh "py_test_percpu" "sudo" "/bcc/tests/python/test_brb.py" test_brb.c -v'
test_brb (__main__.TestBPFSocket) ... skipped 'Missing binaries: neperf, neterver. iperf and netperf packages must be installed.'

----------------------------------------------------------------------
Ran 1 test in 0.000s

OK (skipped=1)
```

tests/python/utils.py

@@ -2,6 +2,7 @@
 from distutils.spawn import find_executable
 import traceback
 import distutils.version
+import shutil
 
 import logging, os, sys
 
@@ -51,6 +52,23 @@ def wrapper(*args, **kwargs):
  return wrapper
  return decorator
 
+# This is a decorator that will skip tests if any binary in the list is not in PATH.
+def skipUnlessHasBinaries(binaries, message):
+ def decorator(func):
+ def wrapper(self, *args, **kwargs):
+ missing = []
+ for binary in binaries:
+ if shutil.which(binary) is None:
+ missing.append(binary)
+
+ if len(missing):
+ missing_binaries = ", ".join(missing)
+ self.skipTest(f"Missing binaries: {missing_binaries}. {message}")
+ else:
+ func(self, *args, **kwargs)
+ return wrapper
+ return decorator
+
 class NSPopenWithCheck(NSPopen):
  """
  A wrapper for NSPopen that additionally checks if the program