Increase time-out from 5 to 15 seconds.

My computer is too slow, so qemuppc tests randomly timed out.  With
this commit, my machine passes the qemuppc tests (if I also edit the
top build.lua to enable qemuppc).

tests/plat/build.lua

@@ -58,7 +58,7 @@ definerule("plat_testsuite",
 					"util/build+testrunner"
 				},
 				commands = {
-					"%{ins[2]} "..e.method.." %{ins[1]} 5 %{ins[3]} > %{outs}; true",
+					"%{ins[2]} "..e.method.." %{ins[1]} 15 %{ins[3]} > %{outs}; true",
 				}
 			}
 		end