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/Makefile.qttest.include

@@ -7,6 +7,7 @@ TESTS += qt/test/test_bitcoin-qt
 
 TEST_QT_MOC_CPP = \
   qt/test/moc_apptests.cpp \
+  qt/test/moc_optiontests.cpp \
   qt/test/moc_rpcnestedtests.cpp \
   qt/test/moc_uritests.cpp
 
@@ -19,6 +20,7 @@ endif # ENABLE_WALLET
 TEST_QT_H = \
   qt/test/addressbooktests.h \
   qt/test/apptests.h \
+  qt/test/optiontests.h \
   qt/test/rpcnestedtests.h \
   qt/test/uritests.h \
   qt/test/util.h \
@@ -30,6 +32,7 @@ qt_test_test_bitcoin_qt_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BITCOIN_
 qt_test_test_bitcoin_qt_SOURCES = \
   init/bitcoin-qt.cpp \
   qt/test/apptests.cpp \
+  qt/test/optiontests.cpp \
   qt/test/rpcnestedtests.cpp \
   qt/test/test_main.cpp \
   qt/test/uritests.cpp \