* test/lisp/eshell/eshell-tests.el
* test/lisp/eshell/em-extpipe-tests.el: Load eshell-tests-helpers with
'require'.
* test/lisp/eshell/eshell-tests-helpers.el (eshell-history-file-name):
Define this here so individual test files don't have to.
* lisp/eshell/em-extpipe.el (eshell-parse-external-pipeline): Fix
misinterpreting sharp-quoted symbols as the beginning of single-quoted
strings (Bug#53518). Add protection against a possible infinite loop.
* test/lisp/eshell/em-extpipe-tests.el (em-extpipe-test-17): New
test (bug#53518).