Refactor Defer and fix detection of 'missing Done' for Defer in Defer

Refactor Defer to make clearer that running the defered function is
just delegated to Cleanup.
The 'Done called' hook is now installed in its own Cleanup.

Note that the panic about missing Done call will now also happen after
the run of the last deferred function. Also it will be correctly
installed multiple times for the weired case where Defer() is called
from a deferred function.

quicktest.go

@@ -109,22 +109,19 @@ func (c *C) Defer(f func()) {
 		// Use TB.Cleanup when available, but add a check
 		// that Done has been called so that we don't run
 		// into unexpected Go version incompatibilities.
-		if c.doneNeeded {
-			// We've already installed the wrapper func that checks for Done
-			// so we can avoid doing it again.
-			cleaner.Cleanup(f)
-			return
+		if !c.doneNeeded {
+			c.doneNeeded = true
+			cleaner.Cleanup(func() {
+				c.mu.Lock()
+				defer c.mu.Unlock()
+				if c.doneNeeded {
+					panic("Done not called after Defer")
+				}
+			})
 		}
-		c.doneNeeded = true
-		cleaner.Cleanup(func() {
-			c.mu.Lock()
-			doneNeeded := c.doneNeeded
-			c.mu.Unlock()
-			if doneNeeded {
-				panic("Done not called after Defer")
-			}
-			f()
-		})
+
+		cleaner.Cleanup(f)
+
 		return
 	}