Add Generic for TestClient.app

[Dreamsorcerer]

This should avoid annoying assert client.app is not None lines in test code.

Fixes #7226.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.42%. Comparing base (63813fe) to head (c3ed458).
Report is 1051 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8977   +/-   ##
=======================================
  Coverage   98.42%   98.42%           
=======================================
  Files         107      107           
  Lines       34681    34707   +26     
  Branches     4110     4114    +4     
=======================================
+ Hits        34134    34160   +26     
  Misses        380      380           
  Partials      167      167           

Flag	Coverage Δ
CI-GHA	98.30% <84.21%> (-0.01%)	⬇️
OS-Linux	97.97% <84.21%> (-0.01%)	⬇️
OS-Windows	96.36% <84.21%> (-0.01%)	⬇️
OS-macOS	97.65% <84.21%> (-0.01%)	⬇️
Py-3.10.11	97.74% <68.42%> (-0.02%)	⬇️
Py-3.10.15	97.67% <68.42%> (-0.02%)	⬇️
Py-3.11.10	97.57% <84.21%> (-0.01%)	⬇️
Py-3.11.9	97.64% <84.21%> (-0.01%)	⬇️
Py-3.12.6	98.02% <84.21%> (-0.01%)	⬇️
Py-3.9.13	97.62% <68.42%> (-0.02%)	⬇️
Py-3.9.20	97.56% <68.42%> (-0.02%)	⬇️
Py-pypy7.3.16	97.18% <68.42%> (-0.02%)	⬇️
VM-macos	97.65% <84.21%> (-0.01%)	⬇️
VM-ubuntu	97.97% <84.21%> (-0.01%)	⬇️
VM-windows	96.36% <84.21%> (-0.01%)	⬇️

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[patchback]

Backport to 3.11: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 6db2c74 on top of patchback/backports/3.11/6db2c747572b3e77a2d64e3f6c5a01ecae74fcff/pr-8977

Backporting merged PR #8977 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.11/6db2c747572b3e77a2d64e3f6c5a01ecae74fcff/pr-8977 upstream/3.11

4. Now, cherry-pick PR Add Generic for TestClient.app #8977 contents into that branch:

   $ git cherry-pick -x 6db2c747572b3e77a2d64e3f6c5a01ecae74fcff

   If it'll yell at you with something like fatal: Commit 6db2c747572b3e77a2d64e3f6c5a01ecae74fcff is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 6db2c747572b3e77a2d64e3f6c5a01ecae74fcff

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add Generic for TestClient.app #8977 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.11/6db2c747572b3e77a2d64e3f6c5a01ecae74fcff/pr-8977

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[bdraco]

There are a lot of conflicts in the backport as I wasn't 100% sure on all of them.

[Dreamsorcerer]

#9245