test: parse the command line arguments in unit tests

Retrieve the command line arguments from boost and pass them to
`BasicTestingSetup` so that we gain extra flexibility of passing any
config options on the test command line, e.g.:

```
test_bitcoin -- -printtoconsole=1 -checkaddrman=5
```

src/test/main.cpp

@@ -11,6 +11,7 @@
 
 #include <test/util/setup_common.h>
 
+#include <functional>
 #include <iostream>
 
 /** Redirect debug log to unit_test.log files */
@@ -24,3 +25,17 @@ const std::function<void(const std::string&)> G_TEST_LOG_FUN = [](const std::str
     if (!should_log) return;
     std::cout << s;
 };
+
+/**
+ * Retrieve the command line arguments from boost.
+ * Allows usage like:
+ * `test_bitcoin --run_test="net_tests/cnode_listen_port" -- -checkaddrman=1 -printtoconsole=1`
+ * which would return `["-checkaddrman=1", "-printtoconsole=1"]`.
+ */
+const std::function<std::vector<const char*>()> G_TEST_COMMAND_LINE_ARGUMENTS = []() {
+    std::vector<const char*> args;
+    for (int i = 1; i < boost::unit_test::framework::master_test_suite().argc; ++i) {
+        args.push_back(boost::unit_test::framework::master_test_suite().argv[i]);
+    }
+    return args;
+};