rpc: make uptime monotonic across NTP jumps

Compute `uptime` from `SteadyClock` so it is unaffected by system time changes after startup.

Derive GUI startup time by subtracting the monotonic uptime from the wall clock time.

Add a functional test covering a large `setmocktime` jump.

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>

src/common/system.cpp

@@ -37,9 +37,6 @@
 
 using util::ReplaceAll;
 
-// Application startup time (used for uptime calculation)
-const int64_t nStartupTime = GetTime();
-
 #ifndef WIN32
 std::string ShellEscape(const std::string& arg)
 {
@@ -130,8 +127,8 @@ std::optional<size_t> GetTotalRAM()
     return std::nullopt;
 }
 
-// Obtain the application startup time (used for uptime calculation)
-int64_t GetStartupTime()
+SteadyClock::duration GetUptime()
 {
-    return nStartupTime;
+    static const auto g_startup_time{SteadyClock::now()};
+    return SteadyClock::now() - g_startup_time;
 }