[css-flexbox] Review tests that fail in all browsers

[gregwhitworth]

The following tests fail in all browsers - review them and submit PR as necessary.

• negative-margins-001.html == reference/negative-margins-001-ref.html
• flexbox_interactive_break-natural.html
• flexbox_computedstyle_flex-basis-0percent.html
• flex-minimum-width-flex-items-005.xht == reference/ref-filled-green-100px-square.xht
• flexbox_computedstyle_align-self-invalid.html
• flex-minimum-width-flex-items-001.xht == reference/ref-filled-green-100px-square.xht
• flexbox_interactive_break-before-item.html
• flexbox_interactive_paged-overflow-2.html
• flex-minimum-width-flex-items-003.xht == reference/ref-filled-green-100px-square.xht
• flexbox_computedstyle_flex-shorthand-number.html
• flexbox_interactive_paged-overflow.html
• flexbox_interactive_break-after-item.html

[gregwhitworth]

I'm going to be dropping in notes while I work through these:

• negative-margins-001.html: was not representing reality - I adjusted this in my previous PR.
• flexbox_interactive_break-natural.html: valid failure for FF/Chrome/Edge
• flexbox_computedstyle_flex-basis-0percent.html: All browsers are returning 0% which was actually specified in the past. Valid failure for all browsers since the spec states it should be 0px.
• flex-minimum-width-flex-items-001.xht: Updated the test to reflect the update of the spec, as it's testing the transferred size and this only checks the cross-axis so I adjusted the test accordingly to accomplish the spirit of the test. Additionally, it was setting a definite width on the flex container and intrinsic minimums are only triggered when min-width|height are set. So modified that as well.
• flexbox_interactive_break-before-item.html: Valid failure on all UAs
• flexbox_interactive_paged-overflow-2.html: This one was a bit weird, I changed the header to not be a fixpos element and changed the text to be more intuitive as most UAs don't paint the background when printing.
• flex-minimum-width-flex-items-003.xht: This actually passes in Edge/Chrome/FF
• flexbox_computedstyle_flex-shorthand-number.html: This is the same as the above - all are returning 0%
• flexbox_interactive_paged-overflow.html: Valid failure on all UAs
• flexbox_interactive_break-after-item.html: Valid failure on all UAs