test: Add tests for GetArg methods / settings.json type coercion

Just add tests. No changes to application behavior. Tests will be
updated in the next commit changing & improving current behavior.

Include a Qt test for GUI startup crash reported by Rspigler in
https://github.com/bitcoin/bitcoin/issues/24457 caused by GetArg
behavior that happens if settings.json contains an integer value for any
of the configuration options which GUI settings can currently clash with
(-dbcache, -par, -spendzeroconfchange, -signer, -upnp, -natpmp, -listen,
-server, -proxy, -proxy, -onion, -onion, -lang, and -prune).

src/qt/test/optiontests.cpp

@@ -0,0 +1,31 @@
+// Copyright (c) 2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <qt/bitcoin.h>
+#include <qt/test/optiontests.h>
+#include <test/util/setup_common.h>
+#include <util/system.h>
+
+#include <QSettings>
+#include <QTest>
+
+#include <univalue.h>
+
+//! Entry point for BitcoinApplication tests.
+void OptionTests::optionTests()
+{
+    // Test regression https://github.com/bitcoin/bitcoin/issues/24457. Check
+    // if setting an integer prune value causes an exception to be thrown in
+    // the OptionsModel constructor.
+    gArgs.LockSettings([&](util::Settings& settings) {
+        settings.forced_settings.erase("prune");
+        settings.rw_settings["prune"] = 3814;
+    });
+    gArgs.WriteSettingsFile();
+    QVERIFY_EXCEPTION_THROWN(OptionsModel{}, std::runtime_error);
+    gArgs.LockSettings([&](util::Settings& settings) {
+        settings.rw_settings.erase("prune");
+    });
+    gArgs.WriteSettingsFile();
+}

src/qt/test/optiontests.h

@@ -0,0 +1,25 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_QT_TEST_OPTIONTESTS_H
+#define BITCOIN_QT_TEST_OPTIONTESTS_H
+
+#include <qt/optionsmodel.h>
+
+#include <QObject>
+
+class OptionTests : public QObject
+{
+    Q_OBJECT
+public:
+    explicit OptionTests(interfaces::Node& node) : m_node(node) {}
+
+private Q_SLOTS:
+    void optionTests();
+
+private:
+    interfaces::Node& m_node;
+};
+
+#endif // BITCOIN_QT_TEST_OPTIONTESTS_H

src/qt/test/test_main.cpp

@@ -10,6 +10,7 @@
 #include <interfaces/node.h>
 #include <qt/bitcoin.h>
 #include <qt/test/apptests.h>
+#include <qt/test/optiontests.h>
 #include <qt/test/rpcnestedtests.h>
 #include <qt/test/uritests.h>
 #include <test/util/setup_common.h>
@@ -89,6 +90,10 @@ int main(int argc, char* argv[])
     if (QTest::qExec(&app_tests) != 0) {
         fInvalid = true;
     }
+    OptionTests options_tests(app.node());
+    if (QTest::qExec(&options_tests) != 0) {
+        fInvalid = true;
+    }
     URITests test1;
     if (QTest::qExec(&test1) != 0) {
         fInvalid = true;