Fix test suite by injecting time dependency for RRSIG validation

[Copilot]

Test suite failing because RRSIG records in test data (from 2019) are validated against current time. All signatures expired years ago, causing 6 test failures.

Changes

Time injection in Resolver

• Added timeNow func() time.Time field to Resolver, defaults to time.Now
• Enables time mocking in tests while preserving production behavior

Updated RRSIG validation

• SignedZone now holds resolver reference to access time function
• verifyRRSIG() uses z.resolver.timeNow() instead of time.Now()
• Made newSignedZone() internal to avoid API surface change

Test updates

• Mock time set to March 7, 2019 12:00 UTC in newResolver() test helper
• Date falls within all test data RRSIG validity periods (inception: Mar 5, expiration: Mar 10+)

// Production: uses real time
resolver, _ := NewResolver("/etc/resolv.conf")

// Tests: inject fixed time
resolver.timeNow = func() time.Time {
    return time.Date(2019, 3, 7, 12, 0, 0, 0, time.UTC)
}

Impact

• Fixes all 6 failing tests without regenerating test data
• No breaking changes to public API
• Coverage unchanged at 77.7%

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fix test suite</issue_title>
<issue_description>Analyze the failing test cases and suggest a strategy to fix them.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Fix test suite #12

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