Add comprehensive test coverage for GTU7 GPS sensor

[Copilot]

Addresses review feedback requesting additional test scenarios beyond the existing RMC precedence test.

Tests Added

• Malformed/invalid NMEA handling: Empty lines, truncated sentences, unknown types, missing fields
• GGA-only operation: Verifies correct position data when RMC/VTG unavailable
• Context cancellation: Clean shutdown between sentence processing
• Multi-constellation variants: GNGGA/GNRMC/GNVTG sentence support (GPS+GLONASS/Galileo)

Example test structure:

func TestGTU7_MalformedSentences(t *testing.T) {
    tests := []struct {
        name  string
        input string
    }{
        {"empty lines", "\n\n\n$GPGGA,..."},
        {"truncated GGA", "$GPGGA,160446.00,3340\n$GPGGA,..."},
        {"invalid sentence type", "$GPXYZ,...\n$GPGGA,..."},
        {"missing lat/lon fields", "$GPGGA,160446.00,,,,,2,08,..."},
    }
    // Verifies driver skips malformed data and processes valid sentences
}

All tests use string readers (hermetic, deterministic) and follow existing timeout patterns.

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Pull request overview

This PR adds comprehensive test coverage for the GTU7 GPS sensor driver to address review feedback. The tests verify the driver's robustness in handling malformed NMEA sentences, operating with limited sentence types, responding to context cancellation, and supporting multi-constellation GNSS formats (GPS+GLONASS/Galileo).

Changes:

• Added tests for malformed/invalid NMEA sentence handling (empty lines, truncated data, unknown types, missing fields)
• Added test for GGA-only operation without RMC/VTG speed data
• Added test for context cancellation behavior
• Added tests for multi-constellation GNSS sentence variants (GNGGA/GNRMC/GNVTG)

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