While implementing SR-11580, I noticed that the filename CustomReflecatbleTests.swift is misspelled. This is a perfect starter bug. The only steps to fix it are to correct the filename and ensure that SwiftSyntax unit tests still pass.
I myself am a newcomer to compiler development, so the following build-script invocation may be suboptimal, but it should work to build Swift and SwiftSyntax (if necessary) and run the SwiftSyntax unit tests. Run this from the parent folder of swift and swift-syntax. Build-and-test output will go to ~/Desktop/allout.txt.