app/fiber: wait till a full event loop iteration ends

fiber.top() fills in statistics every event loop iteration,
so if it was just enabled, fiber.top() may contain 'inf's and
'nan's in fiber cpu usage averages because total time consumed by
the main thread was not yet accounted for.
Same stands for viewing top() results for a freshly created fiber:
its metrics will be zero since it hasn't lived a full ev loop iteration
yet.
Fix this by delaying the test till top() results are meaningful and add
minor refactoring.

Follow-up #2694

test/app/fiber.result

@@ -1469,6 +1469,16 @@ sum = 0
 fiber.top_enable()
 ---
 ...
+function isnan(num)\
+    return num ~= num\
+end
+---
+...
+-- Wait till a full event loop iteration passes, so that
+-- top() contains meaningful results.
+while isnan(fiber.top().cpu["1/sched"].instant) do fiber.yield() end
+---
+...
 a = fiber.top()
 ---
 ...
@@ -1504,9 +1514,9 @@ for k, v in pairs(a) do\
 end
 ---
 ...
-sum_inst
+sum_inst > 99 and sum_inst < 101 or sum_inst
 ---
-- 100
+- true
 ...
 -- not exact due to accumulated integer division errors
 sum_avg > 99 and sum_avg < 101 or sum_avg
@@ -1517,24 +1527,28 @@ tbl = nil
 ---
 ...
 f = fiber.new(function()\
-    for i = 1,1000 do end\
-    fiber.yield()\
-    tbl = fiber.top().cpu[fiber.self().id()..'/'..fiber.self().name()]\
+    local fiber_key = fiber.self().id()..'/'..fiber.self().name()\
+    tbl = fiber.top().cpu[fiber_key]\
+    while tbl.time == 0 do\
+        for i = 1,1000 do end\
+        fiber.yield()\
+        tbl = fiber.top().cpu[fiber_key]\
+    end\
 end)
 ---
 ...
-while f:status() ~= 'dead' do fiber.sleep(0.01) end
+while f:status() ~= 'dead' do fiber.yield() end
 ---
 ...
-tbl["average"] > 0
+tbl.average > 0
 ---
 - true
 ...
-tbl["instant"] > 0
+tbl.instant > 0
 ---
 - true
 ...
-tbl["time"] > 0
+tbl.time > 0
 ---
 - true
 ...

test/app/fiber.test.lua

@@ -634,6 +634,14 @@ sum = 0
 -- gh-2694 fiber.top()
 fiber.top_enable()
 
+function isnan(num)\
+    return num ~= num\
+end
+
+-- Wait till a full event loop iteration passes, so that
+-- top() contains meaningful results.
+while isnan(fiber.top().cpu["1/sched"].instant) do fiber.yield() end
+
 a = fiber.top()
 type(a)
 -- scheduler is present in fiber.top()
@@ -652,19 +660,23 @@ for k, v in pairs(a) do\
     sum_avg = sum_avg + v["average"]\
 end
 
-sum_inst
+sum_inst > 99 and sum_inst < 101 or sum_inst
 -- not exact due to accumulated integer division errors
 sum_avg > 99 and sum_avg < 101 or sum_avg
 tbl = nil
 f = fiber.new(function()\
-    for i = 1,1000 do end\
-    fiber.yield()\
-    tbl = fiber.top().cpu[fiber.self().id()..'/'..fiber.self().name()]\
+    local fiber_key = fiber.self().id()..'/'..fiber.self().name()\
+    tbl = fiber.top().cpu[fiber_key]\
+    while tbl.time == 0 do\
+        for i = 1,1000 do end\
+        fiber.yield()\
+        tbl = fiber.top().cpu[fiber_key]\
+    end\
 end)
-while f:status() ~= 'dead' do fiber.sleep(0.01) end
-tbl["average"] > 0
-tbl["instant"] > 0
-tbl["time"] > 0
+while f:status() ~= 'dead' do fiber.yield() end
+tbl.average > 0
+tbl.instant > 0
+tbl.time > 0
 
 fiber.top_disable()
 fiber.top()