fix: uptime RPC returns 0 on first call

The monotonic uptime fix (#34328) used a function-local static for `g_startup_time`, which was initialized on first `GetUptime()` call instead of app startup time.
This caused the first `uptime()` call to always return 0.

Move `g_startup_time` to namespace scope so it initializes at program start, ensuring the first call returns actual elapsed time. Note that we don't need to make it `static` anymore because it is just used in this single translation unit.

Test was updated to simulate some work before the first call.

Co-authored-by: Carlo Antinarella <carloantinarella@users.noreply.github.com>

src/common/system.cpp

@@ -127,8 +127,8 @@ std::optional<size_t> GetTotalRAM()
     return std::nullopt;
 }
 
-SteadyClock::duration GetUptime()
-{
-    static const auto g_startup_time{SteadyClock::now()};
-    return SteadyClock::now() - g_startup_time;
-}
+namespace {
+    const auto g_startup_time{SteadyClock::now()};
+} // namespace
+
+SteadyClock::duration GetUptime() { return SteadyClock::now() - g_startup_time; }