tests-mbed_drivers-lp_ticker: Adapt tolerance for new Ticker driver

Since this change concerns new ticker driver for NRF51_DK it goas on ticker feature branch.

On master NRF51_DK Ticker driver uses 32kHz RTC for lp and us ticker. Test uses us ticker to measure lp ticker interrupt scheduling (LowPowerTicker.attach), but since the same hardware is used for both tickers the measurement error is constant. On this branch us ticker uses 1 MHz higher precision clock and the results are different - measurement error grows linearly and shows inaccuracy of the RTC.
Test implements constant delta for measured time equal to 2000 us.

Change delta so it depends on lp ticker tested timeout value:
500us for measurement inaccuracy + 5% tolerance for LowPowerTicker

Author: mprse

TESTS/mbed_drivers/lp_ticker/main.cpp

@@ -33,8 +33,9 @@ static const int test_timeout = 10;
 /* Due to poor accuracy of LowPowerTicker on many platforms
    there is no sense to tune tolerance value as it was in Ticker tests.
 
-   Tolerance value is set to 2000us to cover this diversity */
-#define TOLERANCE_US 2000
+   Tolerance value is set to 500us for measurement inaccuracy + 5% tolerance
+   for LowPowerTicker. */
+#define TOLERANCE_US(DELAY) (500 + DELAY / 20)
 
 
 volatile uint32_t ticker_callback_flag;
@@ -117,7 +118,7 @@ void test_multi_call_time(void)
         while(!ticker_callback_flag);
         time_diff = gtimer.read_us();
 
-        TEST_ASSERT_UINT32_WITHIN(TOLERANCE_US, MULTI_TICKER_TIME_MS * 1000, time_diff);
+        TEST_ASSERT_UINT32_WITHIN(TOLERANCE_US(MULTI_TICKER_TIME_MS * 1000), MULTI_TICKER_TIME_MS * 1000, time_diff);
     }
 }
 
@@ -167,7 +168,7 @@ void test_attach_time(void)
     ticker.detach();
     const int time_diff = gtimer.read_us();
 
-    TEST_ASSERT_UINT64_WITHIN(TOLERANCE_US, DELAY_US, time_diff);
+    TEST_ASSERT_UINT64_WITHIN(TOLERANCE_US(DELAY_US), DELAY_US, time_diff);
 }
 
 /** Test single callback time via attach_us
@@ -189,7 +190,7 @@ void test_attach_us_time(void)
     ticker.detach();
     const int time_diff = gtimer.read_us();
 
-    TEST_ASSERT_UINT64_WITHIN(TOLERANCE_US, DELAY_US, time_diff);
+    TEST_ASSERT_UINT64_WITHIN(TOLERANCE_US(DELAY_US), DELAY_US, time_diff);
 }
 
 // Test cases