[fix] TimeVal: add dbg:guard against incorrect subtraction order (#4669)

In principle, any negative subtraction result should be caused by a logical error.

spec/unit/timeval_spec.lua

@@ -0,0 +1,34 @@
+describe("TimeVal module", function()
+    local TimeVal, dbg, dbg_on
+    setup(function()
+        require("commonrequire")
+        TimeVal = require("ui/timeval")
+        dbg = require("dbg")
+        dbg_on = dbg.is_on
+    end)
+
+    after_each(function()
+        if dbg_on then
+            dbg:turnOn()
+        else
+            dbg:turnOff()
+        end
+    end)
+
+    it("should subtract", function()
+        local timev1 = TimeVal:new{ sec = 5, usec = 5000}
+        local timev2 = TimeVal:new{ sec = 10, usec = 6000}
+
+        assert.is.same({sec = 5,usec = 1000}, timev2 - timev1)
+        assert.is.same({sec = -5,usec = -1000}, timev1 - timev2)
+    end)
+
+    it("should guard against reverse subtraction logic", function()
+        dbg:turnOn()
+        TimeVal = package.reload("ui/timeval")
+        local timev1 = TimeVal:new{ sec = 5, usec = 5000}
+        local timev2 = TimeVal:new{ sec = 10, usec = 5000}
+
+        assert.has.errors(function() return timev1 - timev2 end)
+    end)
+end)