crypto/x509: avoid false wildcard matches for 2-label exclusions

[willswire]

When evaluating excluded DNS name constraints against wildcard SANs,
matchDomainConstraint trims one label from both names so that excluded
subdomains like foo.example.com correctly preclude *.example.com.

That trimming was also applied to two-label constraints. For a
constraint like blocked.example, trimming reduces it to example, which
can incorrectly match unrelated wildcard names.

Only apply the wildcard/excluded trimming path when the excluded
constraint has more than two labels.

This change adds regression coverage for the wildcard-matching behavior
in TestExcludedWildcardNameConstraints and adds a chain-level case in
TestConstraintCases.

Updates #76935
Updates #77323

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