crypto/x509: new test TestReadUniqueDirectoryEntries fails on Plan 9

[millerresearch]

CL 776940 added a test which will fail on platforms without symbolic links - for example Plan 9, as seen here:

--- FAIL: TestReadUniqueDirectoryEntries (0.01s)
    root_test.go:331: symlink target-in /tmp/TestReadUniqueDirectoryEntries2849854155/001/link-in: not supported by plan 9
FAIL
FAIL	crypto/x509	108.221s

[gopherbot]

Change https://go.dev/cl/779642 mentions this issue: crypto/x509: skip TestReadUniqueDirectoryEntries if symlinks unsupported

[gabyhelp]

Related Issues

• os: new test TestOpenFileCreateExclDanglingSymlink fails on Plan 9 #73729 (closed)
• cmd/go/internal/fsys: TestWalk_Symlink fails on Plan 9 (which doesn't have symlinks) #41950 (closed)
• cmd/go/internal/fsys: symlink test failing on Plan 9 #41954 (closed)
• x/mod/zip: symlink test failing on Plan 9 #36041 (closed)
• x/net/webdav: TestDir fails on Plan 9 #11453
• cmd/go: TestRemoveDevNull is failing on Plan 9 #17598 (closed)
• os: TestMkdirAll failing Plan 9 #23918 (closed)
• os: TestReadDir failing on Plan 9 since CL 699375 #75287 (closed)
• cmd/link: TestRuntimeTypeDIEs failing on Plan 9 #19944 (closed)
• x/tools/go/packages/packagestest: TestGOPATHExport and TestModulesExport are failing on Plan 9 #46503 (closed)

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