Fixes#1835
If-Match in DeleteObject is a precondition header that compares the client-provided ETag with the server-side ETag before deleting the object. Previously, the comparison failed when the client sent an unquoted ETag, because server ETags are stored with quotes. The implementation now trims quotes from both the input ETag and the server ETag before comparison to avoid mismatches. Both quoted and unquoted ETags are valid according to S3.
All the integration tests used to be in a single file, which had become large, messy, and difficult to maintain. These changes split `tests.go` into multiple files, organized by logical test groups.
