Add documentation on writing tests for Breathe (#931) #925

Job	Run time
build (3.8, 5.0.2)	15s
build (3.8, 5.1.1)	18s
build (3.8, 5.2.3)	17s
build (3.8, 5.3.0)	22s
build (3.8, 6.1.3)	19s
build (3.8, git+https://github.com/sphinx-doc/sphinx.git@master)	14s
build (3.9, 5.0.2)	23s
build (3.9, 5.1.1)	16s
build (3.9, 5.2.3)	19s
build (3.9, 5.3.0)	16s
build (3.9, 6.1.3)	18s
build (3.9, git+https://github.com/sphinx-doc/sphinx.git@master)	13s
build (3.10, 5.0.2)	19s
build (3.10, 5.1.1)	14s
build (3.10, 5.2.3)	19s
build (3.10, 5.3.0)	19s
build (3.10, 6.1.3)	17s
build (3.10, git+https://github.com/sphinx-doc/sphinx.git@master)	17s
build (3.11, 5.0.2)	16s
build (3.11, 5.1.1)	17s
build (3.11, 5.2.3)	16s
build (3.11, 5.3.0)	13s
build (3.11, 6.1.3)	17s
build (3.11, git+https://github.com/sphinx-doc/sphinx.git@master)	14s
build (3.12, 5.0.2)	16s
build (3.12, 5.1.1)	18s
build (3.12, 5.2.3)	15s
build (3.12, 5.3.0)	16s
build (3.12, 6.1.3)	16s
build (3.12, git+https://github.com/sphinx-doc/sphinx.git@master)	21s
	8m 30s

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