bench: specialize working directory name

Since G_TEST_GET_FULL_NAME is not initialized in the benchmark framework,
benchmarks using the unit test setup run in the same directory without
any clear distinction between them.
This poses an extra complication for locating any specific benchmark
directory during a failure.

In master, all benchmarks run in the following path:
/<OS_tmp_dir>/test_common bitcoin/<random_uint256>/

After this commit, benchmarks are contained within its own directory:
/<OS_tmp_dir>/test_common bitcoin/<benchmark_name>/<random_uint256>/

This makes it easier to find any benchmark run when a failure occurs.

src/bench/bench.cpp

@@ -29,7 +29,16 @@ const std::function<void(const std::string&)> G_TEST_LOG_FUN{};
 
 const std::function<std::vector<const char*>()> G_TEST_COMMAND_LINE_ARGUMENTS{};
 
-const std::function<std::string()> G_TEST_GET_FULL_NAME{};
+/**
+ * Retrieve the name of the currently in-use benchmark.
+ * This is applicable only to benchmarks that utilize the unit test
+ * framework context setup (e.g. ones using 'MakeNoLogFileContext<TestingSetup>()').
+ * It places the datadir of each benchmark run within their respective benchmark name.
+ */
+static std::string g_running_benchmark_name;
+const std::function<std::string()> G_TEST_GET_FULL_NAME = []() {
+    return g_running_benchmark_name;
+};
 
 namespace {
 
@@ -117,6 +126,7 @@ void BenchRunner::RunAll(const Args& args)
             bench.output(nullptr);
         }
         bench.name(name);
+        g_running_benchmark_name = name;
         if (args.min_time > 0ms) {
             // convert to nanos before dividing to reduce rounding errors
             std::chrono::nanoseconds min_time_ns = args.min_time;