Bump xunit from 2.5.3 to 2.6.4

[dependabot[bot]]

Bumps xunit from 2.5.3 to 2.6.4.

Commits

• 0f87ff7 v2.6.4
• e466a7c Unit tests for #2850 (v2)
• 0cc026b Add KeyValuePair tests with collections and IEquatable objects in the key slot
• c729d7f Add collection dictionary tests for IEquatable<> objects in the value slot
• 0eb76d2 Attempt to fix race condition reported in visualstudio.xunit/issues#396
• 348c56d Bump up to v2.6.4-pre
• 4c2c193 v2.6.3
• c4e29fd xunit/xunit#2811: Add tests for SortedSet and ImmutableSortedSet overloads fo...
• 51851a6 Unit tests to ensure equality assertions with sets + comparer func throw an e...
• ca68393 Unit tests for #2828 (v2)
• Additional commits viewable in compare view

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[dependabot[bot]]

Superseded by #34.