Don't skip stdin test on Windows

[djdv]

Issues around this were resolved, but the test was never "un-skipped".

C:\...\ipfs\go-ipfs-cmds\cli>systeminfo | findstr /B /C:"OS Version"
OS Version:                10.0.17763 N/A Build 17763

C:\...\ipfs\go-ipfs-cmds\cli>go test -v
=== RUN   TestSynopsisGenerator
--- PASS: TestSynopsisGenerator (0.00s)
    helptext_test.go:27: Synopsis is: cmd [--opt=<OPTION> | -o] [--] <required> [<variadic>...]
=== RUN   TestSameWords
--- PASS: TestSameWords (0.00s)
=== RUN   TestOptionParsing
--- PASS: TestOptionParsing (0.00s)
=== RUN   TestArgumentParsing
--- PASS: TestArgumentParsing (0.00s)
=== RUN   TestBodyArgs
--- PASS: TestBodyArgs (0.00s)
=== RUN   TestCloseWithError
--- PASS: TestCloseWithError (0.00s)
    responseemitter_test.go:88: 0
    responseemitter_test.go:47: stdout:
        ---
        a
        ---
    responseemitter_test.go:48: stderr:
        ---
        Error: some error
        ---
    responseemitter_test.go:88: 1
    responseemitter_test.go:47: stdout:
        ---
        a
        ---
    responseemitter_test.go:48: stderr:
        ---
        Error: some error
        ---
=== RUN   TestSingle
--- PASS: TestSingle (0.00s)
PASS
ok      github.com/ipfs/go-ipfs-cmds/cli        0.048s