Move unittests into a more perceivable directory

unittests/hrt.cpp

@@ -0,0 +1,16 @@
+#include <sys/time.h>
+#include <inttypes.h>
+#include <drivers/drv_hrt.h>
+#include <stdio.h>
+
+hrt_abstime hrt_absolute_time() {
+    struct timeval te; 
+    gettimeofday(&te, NULL); // get current time
+    hrt_abstime us = static_cast<uint64_t>(te.tv_sec) * 1e6 + te.tv_usec; // caculate us
+    return us;
+}
+
+hrt_abstime hrt_elapsed_time(const volatile hrt_abstime *then) {
+	// not thread safe
+    return hrt_absolute_time() - *then;
+}