fix: Include spacing in width for inline horizontal components #2251

[marek-mihok]

The PR fulfills these requirements: (check all the apply)

• It's submitted to the main branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. feat: Add a button #xxx, where "xxx" is the issue number).
• When resolving a specific issue, the PR description includes Closes #xxx, where "xxx" is the issue number.
• If changes were made to ui folder, unit tests (make test) still pass.
• New/updated tests are included

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The problem was that left margin for ui.inline horizontally laid components adds to total component width causing the overflow. Since something like boxSizing: "margin-box" does not exist, the issue is fixed by replacing left margin with left padding and using boxSizing: "border-box" which accounts for both borders and paddings when calculating component dimensions.

This PR also includes a fix for firefox visual regression tests incompatibility with new H2O_WAVE_RECONNECT_TIMEOUT (#2252).

Solution tested across all browsers.
Also both unit and visual regression tests pass.

Closes #2251

[mturoci]

LGTM