nim CI doesn't actually test this package

[timotheecour]

while working on nim-lang/Nim#13642 and debugging resulting CI failures, I noticed that this package isn't actually being tested; before nim-lang/Nim#13642 it silently ignores error, after PR it rightly shows following error:

either a test task needs to be defined or a tests folder needs to exist

2020-03-13T09:47:31.0669762Z FAIL: https://github.com/tulayang/asyncmysql C
2020-03-13T09:47:31.0670240Z Test "https://github.com/tulayang/asyncmysql" in category "nimble-packages"
2020-03-13T09:47:31.0671226Z Failure: reBuildFailed
2020-03-13T09:47:31.0671546Z package test failed
2020-03-13T09:47:31.0671857Z $ nimble test
2020-03-13T09:47:31.0672249Z   Executing task test in d:\a\1\s\pkgstemp\asyncmysql\asyncmysql.nimble
2020-03-13T09:47:31.0672822Z d:\a\1\s\pkgstemp\asyncmysql\config.nims(1, 8) Warning: import os.nim instead; ospaths is deprecated [Deprecated]
2020-03-13T09:47:31.0673500Z d:\a\1\s\pkgstemp\asyncmysql\asyncmysql_1704.nims(5, 22) Warning: imported and not used: 'strutils' [UnusedImport]
2020-03-13T09:47:31.0674251Z oserr.nim(94)            raiseOSError
2020-03-13T09:47:31.0674710Z Error: unhandled exception: The system cannot find the path specified.
2020-03-13T09:47:31.0675204Z Additional info: "d:\\a\\1\\s\\pkgstemp\\asyncmysql\\tests" [OSError]
2020-03-13T09:47:31.0675508Z 
2020-03-13T09:50:59.4182379Z PASS: https://github.com/def-/bigints C                            ( 5.45161009 secs)

if you need more info, see this other answer: flyx/NimYAML#80 (comment)