test: fix flaky 050-ttl.t

We use system clock in the internal implementation for TTL, not
monotonic clock. The test may fail when changing the system clock. In
addition, fiber.sleep() may wake up earlier than planned by the
system clock.

We should to handle these situations in the test.

Closes #187

t/050-ttl.t

@@ -17,13 +17,31 @@ local ttl = 0.1
 
 test:ok(queue, 'queue is loaded')
 
+local function test_take_after_ttl(test, tube, ttl)
+    local attempts, max_attempts = 0, 2
+    local before = fiber.time64()
+    local after = before
+
+    while after - before < ttl * 1000000 and attempts < max_attempts do
+        attempts = attempts + 1
+        fiber.sleep(ttl)
+        after = fiber.time64()
+    end
+
+    if after - before < ttl * 1000000 then
+        test:fail('failed to sleep ttl, is system clock changed?')
+    else
+        test:isnil(tube:take(.1), 'no task is taken')
+    end
+end
+
 test:test('one message per queue ffttl', function (test)
     test:plan(20)
     local tube = queue.create_tube('ompq_ffttl', 'fifottl', { engine = engine })
     for i = 1, 20 do
         tube:put('ompq_' .. i, {ttl=ttl})
-        fiber.sleep(ttl)
-        test:is(#{tube:take(.1)}, 0, 'no task is taken')
+
+        test_take_after_ttl(test, tube, ttl)
     end
 end)
 
@@ -32,8 +50,8 @@ test:test('one message per queue utttl', function (test)
     local tube = queue.create_tube('ompq_utttl', 'utubettl', { engine = engine })
     for i = 1, 20 do
         tube:put('ompq_' .. i, {ttl=ttl})
-        fiber.sleep(ttl)
-        test:is(#{tube:take(.1)}, 0, 'no task is taken')
+
+        test_take_after_ttl(test, tube, ttl)
     end
 end)
 
@@ -42,8 +60,8 @@ test:test('many messages, one queue ffttl', function (test)
     for i = 1, 20 do
         local tube = queue.create_tube('mmpq_ffttl_' .. i, 'fifottl', { engine = engine })
         tube:put('mmpq_' .. i, {ttl=ttl})
-        fiber.sleep(ttl)
-        test:is(#{tube:take(.1)}, 0, 'no task is taken')
+
+        test_take_after_ttl(test, tube, ttl)
     end
 end)
 
@@ -52,8 +70,8 @@ test:test('many messages, one queue utttl', function (test)
     for i = 1, 20 do
         local tube = queue.create_tube('mmpq_utttl_' .. i, 'utubettl', { engine = engine })
         tube:put('mmpq_' .. i, {ttl=ttl})
-        fiber.sleep(ttl)
-        test:is(#{tube:take(.1)}, 0, 'no task is taken')
+
+        test_take_after_ttl(test, tube, ttl)
     end
 end)