timer: Increase the imprecision we accept in this test

On a loaded server we might not be able to reach 10ms precision
in the test configuration and this is not a high precision timer
anyway.
diff --git a/tests/timer/timer_test.c b/tests/timer/timer_test.c
index 3775151..61079bd 100644
--- a/tests/timer/timer_test.c
+++ b/tests/timer/timer_test.c
@@ -60,7 +60,7 @@
 
 /* timer imprecision that we accept for this test: 10 milliseconds. */
 #define TIMER_PRES_SECS		0
-#define TIMER_PRES_USECS	10000
+#define TIMER_PRES_USECS	20000
 
 static int timer_nsteps = MAIN_TIMER_NSTEPS;
 static unsigned int expired_timers = 0;