Factor out time keeping code into its own module: mono_time.c.

It turns out, `unix_time` is also monotonic, and is used as such, so I've
renamed the new functions to `mono_time_*`.

2018-07-08:
```
00:01 <@irungentoo> the idea used to be that the unix_time() function
  could go backward in time but I think I might have started using it like
  if it could not after I changed it so that it would never go back in time
```

1 files changed, 3 insertions, 22 deletions

diff --git a/toxcore/util_test.cc b/toxcore/util_test.cc
index 541aa61f..ff57f3fb 100644
--- a/toxcore/util_test.cc
+++ b/toxcore/util_test.cc
@@ -4,28 +4,7 @@
 
 #include <gtest/gtest.h>
 
-TEST(Util, UnixTimeIncreasesOverTime) {
-  unix_time_update();
-  uint64_t const start = unix_time();
-
-  while (start == unix_time()) {
-    unix_time_update();
-  }
-
-  uint64_t const end = unix_time();
-  EXPECT_GT(end, start);
-}
-
-TEST(Util, IsTimeout) {
-  uint64_t const start = unix_time();
-  EXPECT_FALSE(is_timeout(start, 1));
-
-  while (start == unix_time()) {
-    unix_time_update();
-  }
-
-  EXPECT_TRUE(is_timeout(start, 1));
-}
+namespace {
 
 TEST(Util, TwoRandomIdsAreNotEqual) {
   uint8_t pk1[CRYPTO_PUBLIC_KEY_SIZE];
@@ -49,3 +28,5 @@ TEST(Util, IdCopyMakesKeysEqual) {
 
   EXPECT_TRUE(id_equal(pk1, pk2));
 }
+
+}  // namespace