Flaky Compiler server test: ReferenceCachingCS

[mavasani]

Test failed 3 consecutive times on this PR: #19590

See #19590 (comment)

Runfo Tracking Issue: Flaky Compiler server test: ReferenceCachingCS

Build	Definition	Kind	Run Name

Build Result Summary

Day Hit Count	Week Hit Count	Month Hit Count
0	0	0

[mavasani]

Failed again https://ci.dot.net/job/dotnet_roslyn/job/features_ioperation/job/windows_debug_unit64_prtest/461/

[333fred]

Failed again: https://ci.dot.net/job/dotnet_roslyn/view/Official%20Builds/job/dev15.5.x/job/windows_debug_unit32/58/.

[agocke]

Failed in https://ci.dot.net/job/dotnet_roslyn/job/post-dev15.5-contrib/job/windows_release_unit32_prtest/366/

[jasonmalinowski]

Failed in: https://ci.dot.net/job/dotnet_roslyn/job/master-vs-deps/job/windows_debug_unit64_prtest/575/