Previously, the {Basic,}TestingSetup for fuzzers were set up in many ways:
1. Calling InitializeFuzzingContext, which implicitly constructs a static
const BasicTestingSetup
2. Directly constructing a static const BasicTestingSetup in the initialize_*
function
3. Directly constructing a static TestingSetup and reproducing the
initialization arguments (I'm assuming because
InitializeFuzzingContext only initializes a BasicTestingSetup)
The new, relatively-simple MakeFuzzingContext function allows us to
consolidate these methods of initialization by being flexible enough to
be used in all situations. It:
1. Is templated so that we can choose to initialize any of
the *TestingSetup classes
2. Has sane defaults which are often used in fuzzers but are also
easily overridable
3. Returns a unique_ptr, explicitly transferring ownership to the caller
to deal with according to its situation
