Finish porting all tests from Python to Golang from test_harvester.py

[kvch]

What does this PR do?

This PR finishes porting tests from test_harvester.py. It is based on #24250.

Why is it important?

Two problems were discovered while porting tests:

1. lack of support of trucated files

   The support for stopping reading from truncated files was already implemented. However, filestream input could not start reading it from the beginning.

   A new file system event is added called OpTruncate. When the size of a file has shrinked compared to the last time the scanner has encountered it, an OpTruncate event is emitted. When the prospector gets this event, the HarvesterGroup is restarting the Harvester of the file. Restarting basically means that the new Harvester waits for the previous one to finish and then starts reading the file from the beginning. The wait has a predefined timeout which is not the best solution, but I did not want to introduce waiting indefinitely until the context is cancelled for a Harvester. Although, this might be the right approach...

2. encoding configuration

   Encoding configuration could not be parsed from the configuration. After fixing this, there was not any new problems because most of the underlying code is already covered by unit tests.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
  - [ ] I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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• Build Cause: Pull request #24397 opened

• Start Time: 2021-03-05T16:18:13.219+0000

• Duration: 128 min 21 sec

• Commit: e18e32f

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Test	Results
Failed	0
Passed	13074
Skipped	2215
Total	15289

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Passed	13074
Skipped	2215
Total	15289

[urso]

A new file system event is added called OpTruncate. When the size of a file has shrinked compared to the last time the scanner has encountered it, an OpTruncate event is emitted. When the prospector gets this event, the HarvesterGroup is restarting the Harvester of the file.

Nice. Would it be possible to split the PR into 2 PRs to speed up review and ease discussions? The first PR only adds the new tests that run through. And the second PR with the fix + tests for the 'truncate' problem.

[kvch]

The PR is separated into the following ones:

• Add test for close.reader.after_interval of filestream input #24423
• Add tests and support for truncated files in filestream input #24424
• Add tests for truncated and symlinked files #24425
• Add tests for encoding setting of filestream input #24426