os: use relative paths in a test dir in TestOpenError

Refactor TestOpenError to use relative paths in test cases,
in preparation for extending it to test os.Root.

Use a test temporary directory instead of system directory
with presumed-known contents.

Move the testcase type and case definitions inline with the test.

For #67002

Change-Id: Idc53dd9fcecf763d3e4eb3b4643032e3003d7ef4
Reviewed-on: https://go-review.googlesource.com/c/go/+/620157
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/os/os_test.go

@@ -1777,35 +1777,36 @@ func TestSeekError(t *testing.T) {
 	}
 }
 
-type openErrorTest struct {
-	path  string
-	mode  int
-	error error
-}
-
-var openErrorTests = []openErrorTest{
-	{
-		sfdir + "/no-such-file",
-		O_RDONLY,
-		syscall.ENOENT,
-	},
-	{
-		sfdir,
-		O_WRONLY,
-		syscall.EISDIR,
-	},
-	{
-		sfdir + "/" + sfname + "/no-such-file",
-		O_WRONLY,
-		syscall.ENOTDIR,
-	},
-}
-
 func TestOpenError(t *testing.T) {
 	t.Parallel()
 
-	for _, tt := range openErrorTests {
-		f, err := OpenFile(tt.path, tt.mode, 0)
+	dir := t.TempDir()
+	if err := WriteFile(filepath.Join(dir, "is-a-file"), nil, 0o666); err != nil {
+		t.Fatal(err)
+	}
+	if err := Mkdir(filepath.Join(dir, "is-a-dir"), 0o777); err != nil {
+		t.Fatal(err)
+	}
+
+	for _, tt := range []struct {
+		path  string
+		mode  int
+		error error
+	}{{
+		"no-such-file",
+		O_RDONLY,
+		syscall.ENOENT,
+	}, {
+		"is-a-dir",
+		O_WRONLY,
+		syscall.EISDIR,
+	}, {
+		"is-a-file/no-such-file",
+		O_WRONLY,
+		syscall.ENOTDIR,
+	}} {
+		path := filepath.Join(dir, tt.path)
+		f, err := OpenFile(path, tt.mode, 0)
 		if err == nil {
 			t.Errorf("Open(%q, %d) succeeded", tt.path, tt.mode)
 			f.Close()