test_in_tail: Fix unstable tests

[ashie]

Which issue(s) this PR fixes:
None

What this PR does / why we need it:

• 3fbcc11: The rotation tests are wrong on Windows. sub_test_rotate_file should pass a rotated file to the block, not a new file.
  In addition, it should be opened by Fluent::FileWrapper to avoid locking.
• 90ad37d, bcba06f: Create a new empty directory with a random path on each tests to avoid locking files by Windows OS.
• a065d02: Simplify deleting temp directory. The previous hacky way is no longer needed since we always create a new directory for now.
  In addition, the previous implementations has some bugs, for example FileUtils.rm_f doesn't have secure option even though the latest Ruby (v3.1), probably it intend FileUtils.rm_r.
• 4f3b7f3: Drop CI for Ruby 2.6
• e041b61: Fix some omitted tests on Windows

Docs Changes:
None

Release Note:
Same with the title.

[ashie]

CI fails on all windows versions.
Still something might wrong, I'll check it.

[ashie]

Hmm, it seems that unstable tests are caused by failure of removing previous test files:

2022-06-01T05:30:00.7098380Z     test_emit[flat]:					Failed to clean up D:/a/fluentd/fluentd/test/plugin/../tmp/tail/tail.pos: Permission denied @ rb_file_s_rename - (D:/a/fluentd/fluentd/test/plugin/../tmp/tail/tail.pos, D:/a/_temp/b5cdf7a1924ec6b011fd)
2022-06-01T05:30:01.2203798Z F
2022-06-01T05:30:01.2204492Z ===============================================================================
2022-06-01T05:30:01.2205097Z Failure: test_emit[flat](TailInputTest::singleline)
2022-06-01T05:30:01.2223307Z D:/a/fluentd/fluentd/test/plugin/test_in_tail.rb:474:in `test_emit'
2022-06-01T05:30:01.2223863Z      471: 
2022-06-01T05:30:01.2224264Z      472:       events = d.events
2022-06-01T05:30:01.2224725Z      473:       assert_equal(true, events.length > 0)
2022-06-01T05:30:01.2225182Z   => 474:       assert_equal({"message" => "test3"}, events[0][2])
2022-06-01T05:30:01.2225718Z      475:       assert_equal({"message" => "test4"}, events[1][2])
2022-06-01T05:30:01.2226162Z      476:       assert(events[0][1].is_a?(Fluent::EventTime))
2022-06-01T05:30:01.2226613Z      477:       assert(events[1][1].is_a?(Fluent::EventTime))
2022-06-01T05:30:01.2227027Z <{"message"=>"test3"}> expected but was
2022-06-01T05:30:01.2227379Z <{"message"=>"test1"}>
2022-06-01T05:30:01.2230449Z 
2022-06-01T05:30:01.2230792Z diff:
2022-06-01T05:30:01.2233901Z ? {"message"=>"test3"}
2022-06-01T05:30:01.2234430Z ?                  1  
2022-06-01T05:30:01.2235177Z ?                  ?  
2022-06-01T05:30:01.2235548Z ===============================================================================
2022-06-01T05:30:01.2249202Z Failed to clean up D:/a/fluentd/fluentd/test/plugin/../tmp/tail/tail.pos: Permission denied @ rb_file_s_rename - (D:/a/fluentd/fluentd/test/plugin/../tmp/tail/tail.pos, D:/a/_temp/1953f658c0b72881c77d)
2022-06-01T05:30:01.2260335Z : (0.518041)

[ashie]

All tests succeeded by just one try after a long absence 🎉

• Windows: https://github.com/fluent/fluentd/actions/runs/2420403495
• GNU/Linux: https://github.com/fluent/fluentd/actions/runs/2420403492
• macOS: https://github.com/fluent/fluentd/actions/runs/2420403497

[cosmo0920]

Looks good for fixing unstable tests on Windows.
Still failing macOS CI tasks but these are not related this fix.

[ashie]

Thanks!