test_runner: fix line feed escaping in JUnit #1109

Job	Run time
Build slim tarball	1m 30s
x86_64-linux: with shared libraries	29m 44s
aarch64-linux: with shared libraries	24m 59s
aarch64-darwin: with shared libraries	26m 20s
x86_64-darwin: with shared libraries	55m 20s
	2h 17m 53s

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