os: ReadDir on non-directory doesn't return error on Plan 9

[millerresearch]

CL 699375 fixed a similar issue for Windows, and added a test to TestReadDir which fails on plan9-arm, revealing that Plan 9 has the same problem.

--- FAIL: TestReadDir (0.05s)
    read_test.go:119: ReadDir /tmp/TestReadDir1843189195/001/foo: (nil, ENOTDIR) expected, got ([], <nil>)
FAIL
FAIL	os	80.191s

Plan 9 makes less distinction than other operating systems between directories and regular files, so the ReadDir implementation needs to make an explicit check that its argument is a directory and return ENOTDIR otherwise.

[gabyhelp]

Related Issues

• testing/fstest: test that ReadDir on file returns error, or returns 0 entries if file is empty #44967
• os: TestOpenError fails on some plan9 builders #70440 (closed)
• os: TestMkdirAll failing Plan 9 #23918 (closed)
• os: TestLongPath failing on Plan 9 #17855 (closed)

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[gopherbot]

Change https://go.dev/cl/700855 mentions this issue: os: reject OpenDir of a non-directory file in Plan 9