all: make remaining tests pass on Wine

sed, cat, and diff may not be installed. Rewrite two tests to not use
the first two, and skip the third if diff is not installed.

cmd/gosh/main_test.go

@@ -81,7 +81,7 @@ var interactiveTests = [][]string{
 	{
 		"echo foo |\n",
 		"> ",
-		"cat\n",
+		"read var; echo $var\n",
 		"foo\n",
 	},
 	{

cmd/shfmt/main_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strings"
@@ -49,6 +50,9 @@ func TestStdin(t *testing.T) {
 	})
 
 	t.Run("Diff", func(t *testing.T) {
+		if _, err := exec.LookPath("diff"); err != nil {
+			t.Skip("skipping as the diff tool is not available")
+		}
 		*diff = true
 		defer func() { *diff = false }()
 		in = strings.NewReader(" foo\nbar\n\n")

shell/source_test.go

@@ -37,9 +37,9 @@ var mapTests = []struct {
 		},
 	},
 	{
-		"a=$(echo foo | sed 's/o/a/g')",
+		"a=$(echo foo && echo bar)",
 		map[string]expand.Variable{
-			"a": {Kind: expand.String, Str: "faa"},
+			"a": {Kind: expand.String, Str: "foo\nbar"},
 		},
 	},
 }