More test leaning: a temp-file macro and a match matcher

[bbatsov]

Continues the test-readability pass (items #1 and #2 from the audit).

• projectile-test-with-temp-files - binds vars to fresh temp files or :dir temp directories and removes them on exit (normally or via error), replacing the make-temp-file + unwind-protect + delete-file dance. Applied where it's a clean win (file-only session specs). Most temp-file specs also open buffers and keep their own unwind-protect, so the macro only marginally helps there - I left those rather than risk their cleanup semantics.

• :to-be-a-match-with - a custom matcher checking a projectile-replace--match's fields against a plist in a single expect, naming every field that didn't match; plus projectile-test-find-match. Collapses the multi-expect match-assertion blocks, e.g.

  ;; before: find + 5 separate expects
  (let ((mb (cl-find-if ... projectile-replace--matches)))
    (expect mb :not :to-be nil)
    (expect (projectile-replace--match-line mb) :to-equal 1)
    ... )
  ;; after
  (expect (projectile-test-find-match projectile-replace--matches "mb.txt")
          :to-be-a-match-with '(:line 1 :column 5 :string "foo" :context "café foo bar"))

889 specs green, clean --warnings-as-errors, relint clean.

On the other two audit items: #4 (:to-be-truthy sweep) turns out not to be a safe blanket change - many :to-be t assertions are genuine exact-t checks (e.g. a file-kind marker that is the symbol t), and swapping them to :to-be-truthy would weaken them; I'd only do it case by case. #3 (cl-letf -> spy-on) is a large, delicate churn (relation-test alone has 45) for a readability-only gain, and some cl-letf overrides can't be expressed as spies; I'd do it selectively as its own PR rather than a blanket rewrite. Happy to proceed on either if you'd like.