[css-flexbox] Add a test for negative margin handling

[cbiesinger]

@tabatkins @fantasai please review!

[syncbot]

Automatic validation checks of commit 0b63217 discovered the following problem:

In css-flexbox-1/reference/negative-margins-001-ref.html:

• No author specified.

[syncbot]

Automatic validation checks of commit 7a25a4b passed.

[tabatkins]

^{css-flexbox-1/negative-margins-001.html, line 17 [r1] (raw file):}
Move the border to .container, to make it easier to tell if the flex container is less or greater than 60px.

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Comments from the review on Reviewable.io

[syncbot]

Automatic validation checks of commit 0026c90 passed.

[cbiesinger]

Review status: 0 of 2 files reviewed at latest revision, 1 unresolved discussion, some commit checks pending.

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^{css-flexbox-1/negative-margins-001.html, line 17 [r1] (raw file):}
Done.

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Comments from the review on Reviewable.io

[gsnedders]

Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable

[gsnedders]

Review status: all files reviewed at latest revision, 1 unresolved discussion.

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css-flexbox-1/negative-margins-001.html, line 17 at r1 (raw file):

Previously, cbiesinger (Christian Biesinger) wrote…

Done.

LGTM

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Comments from Reviewable