test: fix flaky vinyl/gc test

The commit fixes the following test failure:

```
[013] vinyl/gc.test.lua                                               [ fail ]
[013]
[013] Test failed! Result content mismatch:
[013] --- vinyl/gc.result	Fri Dec 24 12:27:33 2021
[013] +++ /build/usr/src/debug/tarantool-2.10.0~beta2.18.dev/test/var/rejects/vinyl/gc.reject	Thu Dec 30 10:29:29 2021
[013] @@ -102,7 +102,7 @@
[013]  ...
[013]  check_files_number(2)
[013]  ---
[013] -- true
[013] +- null
[013]  ...
[013]  -- All records should have been purged from the log by now
[013]  -- so we should only keep the previous log file.
```

The reason of the failure is that vylog files are deleted asynchronously
(`box.snapshot()` doesn't wait for `unlink` to complete) since commit
8e429f4b ("wal: remove old xlog files
asynchronously"). So to fix the test, we just need to make the test wait
for garbage collection to complete.

Follow-up #5383

test/vinyl/gc.result

@@ -42,9 +42,11 @@ function ls_vylog() return fio.glob(fio.pathjoin(box.cfg.vinyl_dir, '*.vylog'))
 function gc() temp:auto_increment{} box.snapshot() end
 ---
 ...
-function check_files_number(fnum) return (#files == fnum or \
-    (require('log').error(files) or \
-    require('log').error(box.info.gc()))) end
+function wait_gc(ls_files, expected_count) \
+    return test_run:wait_cond(function() \
+        return #ls_files() == expected_count \
+    end) \
+end
 ---
 ...
 -- Check that run files are deleted by gc.
@@ -60,18 +62,12 @@ while s.index.pk:stat().run_count > 1 do fiber.sleep(0.01) end -- wait for compa
 gc()
 ---
 ...
-files = ls_data()
----
-...
-check_files_number(2)
+assert(wait_gc(ls_data, 2))
 ---
 - true
 ...
 -- Check that gc keeps the current and previous log files.
-files = ls_vylog()
----
-...
-check_files_number(2)
+assert(wait_gc(ls_vylog, 2))
 ---
 - true
 ...
@@ -82,10 +78,7 @@ s:drop()
 gc()
 ---
 ...
-files = ls_data()
----
-...
-check_files_number(0)
+assert(wait_gc(ls_data, 0))
 ---
 - true
 ...
@@ -97,10 +90,7 @@ check_files_number(0)
 gc()
 ---
 ...
-files = ls_vylog()
----
-...
-check_files_number(2)
+assert(wait_gc(ls_vylog, 2))
 ---
 - true
 ...
@@ -109,10 +99,7 @@ check_files_number(2)
 gc()
 ---
 ...
-files = ls_vylog()
----
-...
-check_files_number(1)
+assert(wait_gc(ls_vylog, 1))
 ---
 - true
 ...
@@ -120,10 +107,7 @@ check_files_number(1)
 gc()
 ---
 ...
-files = ls_vylog()
----
-...
-check_files_number(0)
+assert(wait_gc(ls_vylog, 0))
 ---
 - true
 ...

test/vinyl/gc.test.lua

@@ -20,27 +20,26 @@ path = fio.pathjoin(box.cfg.vinyl_dir, tostring(s.id), tostring(s.index.pk.id))
 function ls_data() return fio.glob(fio.pathjoin(path, '*')) end
 function ls_vylog() return fio.glob(fio.pathjoin(box.cfg.vinyl_dir, '*.vylog')) end
 function gc() temp:auto_increment{} box.snapshot() end
-function check_files_number(fnum) return (#files == fnum or \
-    (require('log').error(files) or \
-    require('log').error(box.info.gc()))) end
+function wait_gc(ls_files, expected_count) \
+    return test_run:wait_cond(function() \
+        return #ls_files() == expected_count \
+    end) \
+end
 
 -- Check that run files are deleted by gc.
 s:insert{1} box.snapshot() -- dump
 s:insert{2} box.snapshot() -- dump + compaction
 while s.index.pk:stat().run_count > 1 do fiber.sleep(0.01) end -- wait for compaction
 gc()
-files = ls_data()
-check_files_number(2)
+assert(wait_gc(ls_data, 2))
 
 -- Check that gc keeps the current and previous log files.
-files = ls_vylog()
-check_files_number(2)
+assert(wait_gc(ls_vylog, 2))
 
 -- Check that files left from dropped indexes are deleted by gc.
 s:drop()
 gc()
-files = ls_data()
-check_files_number(0)
+assert(wait_gc(ls_data, 0))
 
 --
 -- Check that vylog files are removed if vinyl is not used.
@@ -49,19 +48,16 @@ check_files_number(0)
 -- This purges records corresponding to dropped runs, but
 -- dropped index records are still stored in vylog.
 gc()
-files = ls_vylog()
-check_files_number(2)
+assert(wait_gc(ls_vylog, 2))
 
 -- All records should have been purged from the log by now
 -- so we should only keep the previous log file.
 gc()
-files = ls_vylog()
-check_files_number(1)
+assert(wait_gc(ls_vylog, 1))
 
 -- The previous log file should be removed by the next gc.
 gc()
-files = ls_vylog()
-check_files_number(0)
+assert(wait_gc(ls_vylog, 0))
 
 temp:drop()