Fix flaky test in `libs/process` (#1314)

## Changes

The order of stdout and stderr being read into the buffer for combined
output is not deterministic due to scheduling of the underlying
goroutines that consume them. That's why this asserts on the contents
and not the order.

libs/process/background_test.go

@@ -1,6 +1,7 @@
 package process
 
 import (
+	"bufio"
 	"bytes"
 	"context"
 	"fmt"
@@ -12,6 +13,17 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func splitLines(b []byte) (lines []string) {
+	scan := bufio.NewScanner(bytes.NewReader(b))
+	for scan.Scan() {
+		line := scan.Text()
+		if line != "" {
+			lines = append(lines, line)
+		}
+	}
+	return lines
+}
+
 func TestBackgroundUnwrapsNotFound(t *testing.T) {
 	ctx := context.Background()
 	_, err := Background(ctx, []string{"/bin/meeecho", "1"})
@@ -46,7 +58,12 @@ func TestBackgroundCombinedOutput(t *testing.T) {
 	}, WithCombinedOutput(&buf))
 	assert.NoError(t, err)
 	assert.Equal(t, "2", strings.TrimSpace(res))
-	assert.Equal(t, "1\n2\n", strings.ReplaceAll(buf.String(), "\r", ""))
+
+	// The order of stdout and stderr being read into the buffer
+	// for combined output is not deterministic due to scheduling
+	// of the underlying goroutines that consume them.
+	// That's why this asserts on the contents and not the order.
+	assert.ElementsMatch(t, []string{"1", "2"}, splitLines(buf.Bytes()))
 }
 
 func TestBackgroundCombinedOutputFailure(t *testing.T) {
@@ -66,10 +83,7 @@ func TestBackgroundCombinedOutputFailure(t *testing.T) {
 		assert.Equal(t, "2", strings.TrimSpace(processErr.Stdout))
 	}
 	assert.Equal(t, "2", strings.TrimSpace(res))
-
-	out := strings.ReplaceAll(buf.String(), "\r", "")
-	assert.Contains(t, out, "1\n")
-	assert.Contains(t, out, "2\n")
+	assert.ElementsMatch(t, []string{"1", "2"}, splitLines(buf.Bytes()))
 }
 
 func TestBackgroundNoStdin(t *testing.T) {