testutil: new package to test run osbuild run functionality

This commit adds a new testutil.MockCommand() helper that will
mock a command in $PATH to allow easier testing of e.g. the
`image-builder-cli build` comamnd that will invoke osbuild.

internal/testutil/testutil_test.go

@@ -0,0 +1,25 @@
+package testutil_test
+
+import (
+	"os/exec"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/osbuild/image-builder-cli/internal/testutil"
+)
+
+func TestMockCommand(t *testing.T) {
+	fakeCmd := testutil.MockCommand(t, "false", "exit 0")
+	defer fakeCmd.Restore()
+
+	err := exec.Command("false", "run1-arg1", "run1-arg2").Run()
+	assert.NoError(t, err)
+	err = exec.Command("false", "run2-arg1", "run2-arg2").Run()
+	assert.NoError(t, err)
+
+	assert.Equal(t, [][]string{
+		{"run1-arg1", "run1-arg2"},
+		{"run2-arg1", "run2-arg2"},
+	}, fakeCmd.Calls())
+}