github-markdown-toc: flush file in test

[stefanb]

The test was flaky, because the file was often empty.

Ref:

• https://stackoverflow.com/a/10222304
• go 1.27rc2, go@1.26 1.26.5 (new formula) #288803
• go 1.26.0, go@1.25 1.25.7 (new formula) #258912

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