Instrument test utility functions to increase fuzzer efficiency

Fuzz Introspector was reporting a high percentage of fuzz blockers
in the `fuzz_diff` test. This means the fuzzing engine was unable
to gain visibility into functions lower in the call stack than the
blocking functions, making it less effective at producing interesting
input data.

This clears a large percentage of the fuzz blockers by adding fuzzer
instrumentation to them via the `@atheris.instrument_func` decorator.

fuzzing/fuzz-targets/fuzz_diff.py

@@ -17,16 +17,19 @@
 class BytesProcessAdapter:
     """Allows bytes to be used as process objects returned by subprocess.Popen."""
 
+    @atheris.instrument_func
     def __init__(self, input_string):
         self.stdout = io.BytesIO(input_string)
         self.stderr = io.BytesIO()
 
+    @atheris.instrument_func
     def wait(self):
         return 0
 
     poll = wait
 
 
+@atheris.instrument_func
 def TestOneInput(data):
     fdp = atheris.FuzzedDataProvider(data)