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, 30 insertions, 0 deletions

diff --git a/toxcore/mono_time_test.cc b/toxcore/mono_time_test.cc
new file mode 100644
index 00000000..5ad91322
--- /dev/null
+++ b/toxcore/mono_time_test.cc
@@ -0,0 +1,30 @@
+#include "mono_time.h"
+
+#include <gtest/gtest.h>
+
+namespace {
+
+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