test(xychart): add focus/blur handler tests

[williaster]

🏠 Internal

• Adds tests for FocusEvent logic added in new(xychart): add support for FocusEvents #959
• improves logic in findNearestGroupDatum to better handle band scale padding (space between bars)

🐛 Bug Fix

Always nice when tests reveal bugs – onFocus handlers weren't being invoked correctly in useEventHandlers, so fixed that in the process.

@hshoff @kristw

[coveralls]

Pull Request Test Coverage Report for Build 408947118

• 17 of 18 (94.44%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 61.919%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/visx-xychart/src/hooks/useEventHandlers.ts	14	15	93.33%

Totals
Change from base Build 408941661:	0.6%
Covered Lines:	1729
Relevant Lines:	2647

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💛 - Coveralls