feat(fxtest): Add WithTestLogger option

[abhinav]

fxtest.New has this really nice behavior where by default,
it uses the testing.TB as the destination for log output.
Unfortunately, fxtest is not good for testing failure cases
because it fails the test if the container failed.

app := fxtest.New(t,
    fx.Invoke(func() error { return errors.New("fail") }),
)

err := app.Start(ctx)
// We never get here because fxtest.New has already failed the test.

So the expectation there is to use fx.New(..) which, by default,
logs to stderr.
This can be addressed by using the fx.WithLogger option
in combination with fxtest.NewTestLogger,
but it ends up being a mouthful:

app := fx.New(
    fx.Invoke(func() error { return errors.New("fail") }),
    fx.WithLogger(func() fxevent.Logger { return fxtest.NewTestLogger(t) }),
)

This PR is a proposal and implementation of a new
fxtest.WithTestLogger option that shortens the above to:

app := fx.New(
    fx.Invoke(func() error { return errors.New("fail") }),
    fxtest.WithTestLogger(t),
)

As an example, a couple tests in Fx itself become more readable
with this new API.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2d14fb7) 98.78% compared to head (7ef57f8) 98.79%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1159   +/-   ##
=======================================
  Coverage   98.78%   98.79%           
=======================================
  Files          30       30           
  Lines        3057     3061    +4     
=======================================
+ Hits         3020     3024    +4     
  Misses         30       30           
  Partials        7        7           

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[JacobOaks]

LGTM!