test: Use tmp files to avoid filesystem permission issues (#59)

Caught during PkgEval run
<https://s3.amazonaws.com/julialang-reports/nanosoldier/pkgeval/by_date/2026-07/05/ASDF.primary.log>

Author: icweaver

test/test-reference.jl

@@ -14,7 +14,7 @@ end
 
 function roundtrip(fpath; extensions = false, validate_checksum = true)
     af = ASDF.load_file(fpath; extensions, validate_checksum)
-    fpath_roundtrip = replace(fpath, ".asdf" => "_roundtrip.asdf")
+    fpath_roundtrip = joinpath(mktempdir(), basename(replace(fpath, ".asdf" => "_roundtrip.asdf")))
     ASDF.write_file(fpath_roundtrip, af.metadata)
     af_roundtrip = ASDF.load_file(fpath_roundtrip; extensions, validate_checksum)
     return af_roundtrip, af