Refactor plugin tests whose assertions don't need to run in on_fit_start hook

[awaelchli]

What does this PR do?

Preparation for #11137

There are a number of tests of this form:

class CB(Callback):
    def on_fit_start(self, trainer):
        assert "something that doesn't require fit()"
        raise Error()

trainer = Trainer(callbacks=CB()) 
with pytest.raises(Error):
    trainer.fit()

These can actually be written without relying on the on_fit_start hook, like so:

trainer = Trainer(callbacks=CB()) 
assert "something that doesn't require fit()"

The tests are a prehistoric artifact from a time when many accelerator related settings like global_rank etc. were not available upon trainer instantiation.

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @justusschock @awaelchli @akihironitta @Borda

[codecov]

Codecov Report

Merging #11149 (b1b0cdd) into master (62f1e82) will increase coverage by 4%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #11149    +/-   ##
========================================
+ Coverage      88%      92%    +4%     
========================================
  Files         177      177            
  Lines       16526    16551    +25     
========================================
+ Hits        14580    15218   +638     
+ Misses       1946     1333   -613