Enhance tests in diktat

[nulls]

What's done:

• removed unused imports in tests
• removed deleting temp files in tests
• added checking assertion for content and status

It's part of #1769

[codecov[bot]]

Codecov Report

Merging #1780 (01bc5bf) into master (d7fbb5a) will not change coverage. The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master    #1780   +/-   ##
=========================================
  Coverage     78.25%   78.25%           
  Complexity     2409     2409           
=========================================
  Files           126      126           
  Lines          8453     8453           
  Branches       2149     2149           
=========================================
  Hits           6615     6615           
  Misses          880      880           
  Partials        958      958           

Files	Coverage Δ
...m/saveourtool/diktat/ruleset/utils/AstNodeUtils.kt	80.62% <0.00%> (ø)

[github-actions[bot]]

JUnit Tests (Windows, EnricoMi/publish-unit-test-result-action@v2)

   164 files  ±0     164 suites  ±0   4m 34s :stopwatch: - 1m 59s 1 388 tests ±0  1 357 :heavy_check_mark: ±0  31 :zzz: ±0  0 :x: ±0  2 767 runs  ±0  2 736 :heavy_check_mark: ±0  31 :zzz: ±0  0 :x: ±0 

Results for commit 01bc5bf8. ± Comparison against base commit d7fbb5aa.

[github-actions[bot]]

JUnit Tests (macOS, EnricoMi/publish-unit-test-result-action@v2)

   164 files  ±0     164 suites  ±0   11m 6s :stopwatch: + 7m 8s 1 388 tests ±0  1 345 :heavy_check_mark: ±0  43 :zzz: ±0  0 :x: ±0  2 767 runs  ±0  2 724 :heavy_check_mark: ±0  43 :zzz: ±0  0 :x: ±0 

Results for commit 01bc5bf8. ± Comparison against base commit d7fbb5aa.