refactor: reduce sqlparse only test cases

[reata]

Closes #347
Closes #348
Closes #408

[codecov]

Codecov Report

Merging #414 (64f5b15) into master (4be0c80) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #414   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files          42       42           
  Lines        2013     2021    +8     
=======================================
+ Hits         2001     2009    +8     
  Misses         12       12           

Files Changed	Coverage Δ
.../parser/sqlfluff/extractors/dml_merge_extractor.py	100.00% <ø> (ø)
...parser/sqlfluff/extractors/dml_insert_extractor.py	100.00% <100.00%> (ø)
sqllineage/core/parser/sqlfluff/utils.py	98.22% <100.00%> (+0.04%)	⬆️