Implement assert matches with assert on

[philderbeast]

A follow on from #10646 that I had to rebase this on top of changes from #10647 that added assertOutputMatches and assertOutputDoesNotMatch to cabal-testsuite/src/Test/Cabal/Prelude.hs. At that time I made this small change but saw that assertOutputMatches and assertOutputDoesNotMatch could be implemented with assertOn. That what this pull request does.

-    unless (concatOutput output =~ regex) $
+    unless (encodeLf output =~ regex) $

---

Template B: This PR does not modify behaviour or interface

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

QA Notes

I've moved tests to cabal-testsuite/AssertTests/cabal.test.hs that are only flexing the assert* functions and are not truly package tests. These tests pass but you'll want to see them fail too to see that the output is as expected. To do that you'll need to make a change and rerun the test:

$ git diff
diff --git a/cabal-testsuite/AssertTests/cabal.test.hs b/cabal-testsuite/AssertTests/cabal.test.hs
index d819c99cc..b44acd2e4 100644
--- a/cabal-testsuite/AssertTests/cabal.test.hs
+++ b/cabal-testsuite/AssertTests/cabal.test.hs
@@ -29,7 +29,7 @@ main = cabalTest . recordMode RecordMarked $ do
   assertOutputMatches " errors " out
   assertOutputDoesNotMatch " error " out
 
-  assertOutputMatches "[[:space:]]+errors[[:space:]]+" out
+  assertOutputMatches "[[:space:]]+ewwows[[:space:]]+" out
   assertOutputDoesNotMatch "[[:space:]]+error[[:space:]]+" out
 
   log "Pseudo multiline string marking:"

$ cabal run cabal-testsuite:cabal-tests -- --with-cabal=./dist-newstyle/build/x86_64-linux/ghc-9.10.1/cabal-install-3.15.0.0/x/cabal/build/cabal/cabal cabal-testsuite/AssertTests/cabal.test.hs
...
# STDERR:
cabal.test.hs: expected:
regex match with '[[:space:]]+ewwows[[:space:]]+'
CallStack (from HasCallStack):
  assertOn, called at src/Test/Cabal/Prelude.hs:835:23 in cabal-testsuite-3-inplace:Test.Cabal.Prelude
  assertOutputMatches, called at cabal-testsuite/AssertTests/cabal.test.hs:32:3 in main:Main

[9999years]

I think this is good. Longer-term, I want better exception-throwing infrastructure that makes it possible to use the normal expect-test and output-marker machinery for these tests.

[philderbeast]

the normal expect-test and output-marker machinery for these tests.

Thanks for the approval @9999years. Could you please link to or describe what you mean by the above?

[9999years]

@philderbeast I'll put some more details here soon, but here's the general idea:
#10747

[Mikolaj]

@philderbeast: could you please restart CI and set the merge label? thank you for the PR!

[philderbeast]

Label merge+no rebase is necessary when the pull request is from an organisation.