test: test_bitcoin: allow -testdatadir=<datadir>

Specifying this argument overrides the path location for test_bitcoin;
it becomes <datadir>/test_common_Bitcoin Core/<testname>/datadir. Also,
this directory isn't removed after the test completes. This can make it
easier for developers to study the results of a test (see the state of
the data directory after the test runs), and also (for example) have an
editor open on debug.log to monitor it across multiple test runs instead
of having to re-open a different pathname each time.

Example usage (note the "--" is needed):

test_bitcoin --run_test=getarg_tests/boolarg -- \
-testdatadir=/somewhere/mydatadir

This will create (if necessary) and use the data directory:

/somewhere/mydatadir/test_common_Bitcoin Core/getarg_tests/boolarg/datadir

Co-authored-by: furszy <mfurszy@protonmail.com>

src/test/fuzz/fuzz.cpp

@@ -35,6 +35,8 @@ __AFL_FUZZ_INIT();
 
 const std::function<void(const std::string&)> G_TEST_LOG_FUN{};
 
+const std::function<std::string()> G_TEST_GET_FULL_NAME{};
+
 /**
  * A copy of the command line arguments that start with `--`.
  * First `LLVMFuzzerInitialize()` is called, which saves the arguments to `g_args`.