新增test_connection方法，用于检测实例连接

[hhyo]

关联issue: #694 #1451 #1667

[LeoQuote]

测试应该可以挪到新的地方去，不用删除吧

[codecov]

Codecov Report

Merging #1670 (1d84467) into master (1f00cd8) will decrease coverage by 0.02%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #1670      +/-   ##
==========================================
- Coverage   76.23%   76.21%   -0.03%     
==========================================
  Files          91       91              
  Lines       14300    14293       -7     
==========================================
- Hits        10902    10893       -9     
- Misses       3398     3400       +2     

Impacted Files	Coverage Δ
common/tests.py	100.00% <ø> (ø)
sql/urls.py	100.00% <ø> (ø)
sql/engines/redis.py	75.92% <25.00%> (-0.49%)	⬇️
sql/engines/__init__.py	67.03% <33.33%> (-3.21%)	⬇️
sql/engines/mongo.py	50.00% <50.00%> (ø)
common/check.py	86.36% <100.00%> (+2.55%)	⬆️
sql/engines/clickhouse.py	71.48% <100.00%> (+0.11%)	⬆️
sql/engines/goinception.py	65.87% <100.00%> (+0.16%)	⬆️
sql/engines/mssql.py	70.20% <100.00%> (+0.15%)	⬆️
sql/engines/mysql.py	82.83% <100.00%> (+0.05%)	⬆️
... and 3 more

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[hhyo]

测试应该可以挪到新的地方去，不用删除吧

前面没有剔除干净的Inception代码，已经不使用了