fix: cte inside bracketed of insert

[duongphannamhung]

re-fix issue: #377

[codecov]

Codecov Report

Merging #395 (701c80a) into master (52f3b75) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #395   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files          42       42           
  Lines        2008     2013    +5     
=======================================
+ Hits         1996     2001    +5     
  Misses         12       12           

Impacted Files	Coverage Δ
...parser/sqlfluff/extractors/dml_insert_extractor.py	100.00% <100.00%> (ø)

[reata]

I have removed the is_cte function as it doesn't seem that we can reuse it. Also the test case is moved to test_sqlfluff.py, which contains all the new features we don't/won't support in sqlparse. Other than that, this PR is perfect fine.