[R-package][tests] remove uses of testthat::expect_is()

[jameslamb]

Similar to #4915, this PR proposes removing {lightgbm}'s use of a {testthat} function that is currently deprecated.

From the vignette describing {testthat} 3.x (link), which was first released in October 2020 (link)

expect_is() is deprecated in favour of the more specific expect_type(), expect_s3_class(), and expect_s4_class().
This ensures that you check the expected class along with the expected OO system.

This PR proposes removing uses of expect_is() in the R package's tests. It proposes replacing them with expect_true(methods::is()) instead of the newest {testthat} because those 3 methods can't be used to test the class of an R6 object.

[StrikerRUS]

Thanks for the updates!

[github-actions]

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