bionic: test: time()

Validate return values from time(), confirm progression.

Test: bionic-unit-tests --gtest_filter=time.time
Bug: 63737556
Change-Id: I08186780c42ded9648b3e788bb2caa2210d00dab
diff --git a/tests/time_test.cpp b/tests/time_test.cpp
index 7a96760..fccff67 100644
--- a/tests/time_test.cpp
+++ b/tests/time_test.cpp
@@ -31,6 +31,31 @@
 
 #include "private/bionic_constants.h"
 
+TEST(time, time) {
+  // Acquire time
+  time_t p1, t1 = time(&p1);
+  // valid?
+  ASSERT_NE(static_cast<time_t>(0), t1);
+  ASSERT_NE(static_cast<time_t>(-1), t1);
+  ASSERT_EQ(p1, t1);
+
+  // Acquire time one+ second later
+  usleep(1010000);
+  time_t p2, t2 = time(&p2);
+  // valid?
+  ASSERT_NE(static_cast<time_t>(0), t2);
+  ASSERT_NE(static_cast<time_t>(-1), t2);
+  ASSERT_EQ(p2, t2);
+
+  // Expect time progression
+  ASSERT_LT(p1, p2);
+  ASSERT_LE(t2 - t1, static_cast<time_t>(2));
+
+  // Expect nullptr call to produce same results
+  ASSERT_LE(t2, time(nullptr));
+  ASSERT_LE(time(nullptr) - t2, static_cast<time_t>(1));
+}
+
 TEST(time, gmtime) {
   time_t t = 0;
   tm* broken_down = gmtime(&t);