[sw rv_timer test] Minor fix and updates

- Fixes `kDeadline` value to correctly reflect 10ms (instead of
65.536ms)
- Adds another check at the end of the test to ensure the timer reads
an expected time value that is within a safe bound.

Signed-off-by: Srikrishna Iyer <sriyer@google.com>

sw/device/tests/dif/dif_rv_timer_sanitytest.c

diff --git a/sw/device/tests/dif/dif_rv_timer_sanitytest.c b/sw/device/tests/dif/dif_rv_timer_sanitytest.c
index 9bbe94a..1b0fc88 100644
--- a/sw/device/tests/dif/dif_rv_timer_sanitytest.c
+++ b/sw/device/tests/dif/dif_rv_timer_sanitytest.c
@@ -28,7 +28,7 @@
 static const uint32_t kComparator = 0;
 
 static const uint64_t kTickFreqHz = 1000 * 1000;  // 1 MHz.
-static const uint64_t kDeadline = 0x10000;        // 10 ms.
+static const uint64_t kDeadline = 30000;          // 30 ms.
 
 static void test_handler(void) {
   CHECK(!irq_fired, "Entered IRQ handler, but `irq_fired` was not false!");