5521: bundler gem: Support test-unit as a testing framework r=hsbt a=kou



Co-authored-by: Kouhei Sutou <kou@clear-code.com>

Previously, we were recommending `bundle config deployment true; bundle
install` as an alternative to `bundle install --deployment`, but they
were not working equally.

We might be shelling out to rubygems in a `bundle exec` context. In the
case where we don't shell out to the `gem` binstub directly, the
previous trick wouldn't work.

Instead, reset the rubygems ui right after `bundler/setup`, so that if
we end up shelling out to rubygems, it will use its default shell
(non-silent).

The best place to fix this would probably be right inside the `gem`
script, but even if we fix it there, we'll need workarounds for previous
rubygems versions inside `bundler` so I think this is good for now.

7423: Ignore local overrides for bundle pristine r=deivid-rodriguez a=fatkodima

Closes #6836

Co-authored-by: fatkodima <fatkodima123@gmail.com>

6455: [CLI::Gem] Add a --rubocop option r=deivid-rodriguez a=segiddins

Based upon #6451.

### What was the end-user problem that led to this PR?

The problem was I always have to remember how to add RuboCop to my Rakefile when I set up a new gem.

### What was your diagnosis of the problem?

My diagnosis was that RuboCop has become enough of a community standard that it makes sense to offer it in `bundle gem`.

### What is your fix for the problem, implemented in this PR?

My fix adds an option to `bundle gem` to add in RuboCop, in the same way options like `:coc`and `:mit` are handled.

### Why did you choose this fix out of the possible options?

I chose this fix because it does not require bundler have an opinion on _how_ rubocop is configured.

Co-authored-by: Samuel Giddins <segiddins@segiddins.me>

7393: Make 'bundle add' cache newly added gems when needed r=deivid-rodriguez a=andrehjr

### What was the end-user problem that led to this PR?

The problem was that when calling `bundle add` with cache_all as true, I have to call an additional `bundle install` to vendor gems.

### What was your diagnosis of the problem?

My diagnosis was that a call to Bundler.load.cache was missing. For example Bundler::CLI::Update does the same after installing gems.

### What is your fix for the problem, implemented in this PR?

My fix was to call Bundler.load.cache when `Bundler.app_cache.exist? ` 

### Why did you choose this fix out of the possible options?

I chose this fix because it looks like this makes the behavior consistent with other commands.

I should also say that, as this is my first PR here, I'm not sure that this is the best solution, and it seems an easy fix for #7384. 

Co-authored-by: André Luis Leal Cardoso Junior <andrehjr@gmail.com>