refactor(test): reduce cyclomatic complexity in test helpers to reach goreportcard 100% gocyclo

Extract inline closures and large sub-test bodies into package-level
helper functions across four test files, bringing all flagged functions
below the gocyclo threshold of 15:

- metadata_test.go: TestMetadataSetters (41→<15) — 15 testSetXxx helpers
- read_test.go: TestMetadata_NilMetadata (27→<15) — assertNilStringAccessors + assertNilBoolAccessors
- metadata_convenience_test.go: buildTIFFMultiIFD (26→<15) — tiffPlaceEntries + tiffWriteIFD
- metadata_convenience_test.go: TestMetadata_ExifIFDMultipleTagsCoexist (18→<15) — assert helpers
- exif/exif_test.go: buildCameraEXIF (18→<15) — split into entry-builder functions
- exif/exif_test.go: TestEXIFSetters (23→<15) — testEXIFSetXxx helpers + newMinimalEXIF + exifRoundTrip

No production code changed. All 25 packages pass go test ./...

Author: FlavioCFOliveira