Correctly include fixed strings before a prefix wildcard in PCM

Correctly include fixed strings before a prefix wildcard in PCM

In 03ac16ece40ba3e3ba805d6a61cc457d84bf3792 I fixed a bug with the
PCM implementation of substring completion, relating to the handling
of PCM wildcards.
However, this fix was incomplete.  This change completes the fix by
also including a fixed string if it appears before a 'prefix'
wildcard, even if 'try-completion' doesn't discover that fixed
string grows to a unique completion.
I discovered this bug while working on enhancements to PCM
completion related to 'completion-pcm-leading-wildcard'.
* lisp/minibuffer.el (completion-pcm--merge-completions): Include
fixed strings before 'prefix wildcard.  (Bug#72819)
* test/lisp/minibuffer-tests.el (completion-substring-test-5): Add a
test for this behavior.