Add .fromEnvironment flags to the global declarer

[johnniwinther]

The global declarer is used by package:test_reflective_loader which has no means of configuring the test output. It is for instance used to run all tests in package:analyzer, and the current output prints a line for each test (of which there is currently >32000) and uses colors in the output. This makes it very hard to output to find failing tests, since the failures are hidden within all the succeeding tests and piping the output to a file for easier search still leaves the color in the output.

This change adds .fromEnvironment variables to the creation of the global declarer, such that it can be configured at the command line. This allow for enabling compact output which only displays the failing tests, and whether to use colors in the output.

[github-actions]

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[kevmoo]

Should be a bit more explicit here?

It'd also be nice to update this section of the readme – so this feature isn't lost.

https://github.com/dart-lang/test/tree/master/pkgs/test#selecting-a-test-reporter

I have a few packages where I'd use this, too!

[jakemac53]

There are many more issues than just this with running tests directly as opposed to through the test runner - test annotations won't be respected and test configuration files also won't be respected (and probably many more such as timeouts etc).

Is it possible to just use the test runner here?

[jakemac53]

I also think it is potentially confusing that these environment variables are only supported by the global declarer and don't more generally set the defaults of test reporters as a whole.