os/exec: TestCommand fails on Windows when `NoDefaultCurrentDirectoryInExePath` is set in the environment

[bcmills]

In investigating the data race reported in https://go.dev/cl/527337, I looked at the existing tests that cover the call to lookExtensions is exec.Cmd.Start on Windows.

I found several test cases in TestCommand that look suspicious: they expect exec.LookPath to always resolve a bare filename relative to the current directory, possibly with an ErrDot error. However, if NoDefaultCurrentDirectoryInExePath is set in the environment, exec.LookPath does not even attempt to resolve in the current directory, so the tests presumably won't pass.

Running the tests on a gomote confirms that hypothesis as of https://go.dev/cl/527337:

gopher@GOLANG-BUILDLET C:\workdir\go\src>set NoDefaultCurrentDirectoryInExePath=TRUE

gopher@GOLANG-BUILDLET C:\workdir\go\src>..\bin\go test os/exec
--- FAIL: TestCommand (0.58s)
    --- FAIL: TestCommand/2 (0.06s)
        lp_windows_test.go:396: test={PATH:p2;p files:[a.exe p\a.exe p2\a.exe] dir: arg
0:a want:a.exe fails:false}: want "a.exe", got "p2\\a.exe"
    --- FAIL: TestCommand/10 (0.07s)
        lp_windows_test.go:396: test={PATH:p2;p files:[a.exe p\a.exe p2\a.exe] dir:p ar
g0:a.exe want:p\a.exe fails:false}: want "p\\a.exe", got "p2\\a.exe"
    --- FAIL: TestCommand/11 (0.06s)
        lp_windows_test.go:396: test={PATH:p2;p files:[a.exe p\a.exe p2\a.exe] dir:p ar
g0:a want:p\a.exe fails:false}: want "p\\a.exe", got "p2\\a.exe"
    --- FAIL: TestCommand/1 (0.06s)
        lp_windows_test.go:396: test={PATH:p2;p files:[a.exe p\a.exe p2\a.exe] dir: arg
0:a.exe want:a.exe fails:false}: want "a.exe", got "p2\\a.exe"
FAIL
FAIL    os/exec 0.947s
FAIL

(CC @golang/windows, @ianlancetaylor)

[gopherbot]

Change https://go.dev/cl/528035 mentions this issue: os/exec: simplify Windows-specific tests