eshell-next-prompt: More precisely navigate to the prompt (bug#63748)

eshell-next-prompt: More precisely navigate to the prompt (bug#63748)

* lisp/eshell/em-prompt.el (eshell-next-prompt): Navigate to the
current prompt more accurately by using text properties instead of
going to the beginning of the line.  This is important for multiline
prompts, as they don't necessarily start at the beginning of the
current line.

* test/lisp/eshell/em-prompt-tests.el
(em-prompt-test--with-multiline):
Execute a given body with a multiline prompt.

(em-prompt-test/next-previous-prompt-with):
(em-prompt-test/forward-backward-matching-input-with):
Helper functions for code reuse.

(em-prompt-test/forward-backward-matching-input):
(em-prompt-test/next-previous-prompt):
Rewrite in terms of the appropriate helper functions.

(em-prompt-test/next-previous-prompt-multiline):
(em-prompt-test/forward-backward-matching-input-multiline):
Add multiline variants of existing tests.