Replace expect_equal() with expect_identical() in tests where possible

[peterdesmet]

Note from Pieter:

reference: https://testthat.r-lib.org/reference/equality-expectations.html

These functions provide two levels of strictness when comparing a computation to a reference value. expect_identical() is the baseline; expect_equal() relaxes the test to ignore small numeric differences.

In the 3rd edition, both functions use waldo. They differ only in that expect_equal() sets tolerance = testthat_tolerance() so that small floating point differences are ignored; this also implies that (e.g.) 1 and 1L are treated as equal.