Fix behavior of 'eshell-hist-ignoredups' when set to 'erase'

Fix behavior of 'eshell-hist-ignoredups' when set to 'erase'

* lisp/eshell/em-hist.el (eshell-add-input-to-history): Refactor to
use 'pcase' and correct the logic for the 'erase' case.

* test/lisp/eshell/em-hist-tests.el: Require our test helpers.
(eshell-write-readonly-history): Rename to...
(em-hist-test/write-readonly-history): ... this.
(em-hist-test/add-to-history/allow-dups)
(em-hist-test/add-to-history/no-consecutive-dups)
(em-hist-test/add-to-history/erase-dups): New tests (bug#63360).