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/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;