Incorrect way to handle directory read/write access checks in the test command

[ic-it]

Problem Description:
The current implementation of the TsWrite/TsRead case in the code does not correctly determine if a directory is writable/readable.
Specifically, the following snippet:

sh/interp/test.go

Lines 158 to 163 in 86363cf

	case syntax.TsWrite:
	f, err := r.open(ctx, x, os.O_WRONLY, 0, false)
	if err == nil {
	f.Close()
	}
	return err == nil

will fail when x refers to a directory because opening a directory with the os.O_WRONLY flag is not a valid operation. Consequently, the function will always return false for directories, even if they are writable.

[ic-it]

In this case, I suggest, use these solutions from here

Since there is no single and cross-platform solution, we can do it differently for different systems via +build

There is also an option to create a file to "check" if it can be written to the directory. But this approach is IMHO error-prone

[mvdan]

We already have some OS-dependent logic via os_unix.go and os_notunix.go. It would be perfectly fine to extend this to perform better on unix as you suggest; we would just need tests to cover the edge case above.