Backend.Tests: move SetUp to Deserialization too #2946

Job	Run time
macOS--dotnet6-and-mono	2m 34s
macOS--dotnet6-only	3m 40s
macOS--mono-only	6m 3s
windows--dotnet6-and-legacyFramework	3m 27s
linux22-github--dotnet-and-mono	1m 40s
linux22-github--dotnet-and-newmono	2m 20s
windows--dotnet6-only	3m 48s
windows--legacyFramework-only	12m 50s
linux22-vanilla--newmono-only	5m 44s
linux22-vanilla--stockdotnet6-and-newmono	2m 47s
linux22-vanilla--stockdotnet6-and-stockmono	2m 13s
linux22-vanilla--stockdotnet6-only	2m 6s
linux22-vanilla--stockmono-only	4m 45s
linux20-github--dotnet-and-mono	1m 46s
linux20-github--dotnet-and-newmono	1m 55s
linux20-vanilla--newmono-only	5m 28s
linux20-vanilla--stockmono-only	4m 45s
conventions	0s
snap_pkg	0s
	1h 7m 51s

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