[with-temp] Delay pprint in do-with-temp

[alexander-yakushev]

Using pprint to provide context for clojure.core/testing macro can be a bit expensive and ultimately wasteful if the test doesn't fail. I've merged a fix for this in Metabase here metabase/metabase#52830, but I've just come up with a better idea. Since clojure.test ultimately calls str on the values inside clojure.test/*testing-contexts*, we can pass a custom CharSequence object to testing that doesn't compute the result until requested.

Here's the effect it has on one of the slow tests, metabase.api.downloads-exports-test/pivot-with-scale-test: https://flamebin.dev/cQgqtG. -10% allocations and comparative speed improvement in that test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.55%. Comparing base (b412026) to head (80d06fc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #197   +/-   ##
=======================================
  Coverage   83.55%   83.55%           
=======================================
  Files          37       37           
  Lines        2506     2506           
  Branches      212      212           
=======================================
  Hits         2094     2094           
  Misses        200      200           
  Partials      212      212           

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