fix(form-field): remove nonbreaking space before * for required fields

[stevenyxu]

The surrounding .mat-form-field-label is already white-space: nowrap so the   doesn't serve any
wrap-preventing function. But it does prevent any trailing spacing from collapsing with
the space before the asterisk, causing for a bad visual appearance of a double space in these cases.

Addresses #15489.

I attempted to use a literal space and not the symbol, but the compiler collapsed it for me.

[stevenyxu]

I tried to add a test to check in a more deliberate way that the spaces were deliberate and there's no real DOM API I found to get the post-collapsing rendered text short of doing a screendiff test. Both innerText and textContent give the spaces pre-collapsing. I did adjust the test to match explicitly literal space and not \s which also matches non-breaking space, so the test would have failed without the change.

I did not add an explicit test that puts a trailing space in the placeholder or uses a mat-label because I wasn't sure it was worth a test. I have a change ready for it so if you want me to add it, let me know.

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
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     status "ci/circleci: tests_browserstack" is failing

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[mmalerba]

@stevenyxu looks like there's some failing tests

[stevenyxu]

@mmalerba sorry about that. Fixed and rebased today.

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