fix: allow for newlines in expectedExitCode.txt (#1987)

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**What type of PR is this?**

Bug fix

**What package or component does this PR mostly affect?**

testtools

**What does this PR do? Why is it needed?**

Small fix to allow newlines in `expectedExitCode.txt` files which can be
added automatically by formatters.

**Which issues(s) does this PR fix?**

N/A

**Other notes for review**

testtools/files.go

@@ -236,7 +236,7 @@ func TestGazelleGenerationOnPath(t *testing.T, args *TestGazelleGenerationArgs)
 				return nil
 			}
 			if d.Name() == expectedExitCodeFilename {
-				config.ExitCode, err = strconv.Atoi(string(content))
+				config.ExitCode, err = strconv.Atoi(strings.TrimSpace(string(content)))
 				if err != nil {
 					// Set the ExitCode to a sentinel value (-1) to ensure that if the caller is updating the files on disk the value is updated.
 					config.ExitCode = -1