[build-presets] Skip libdispatch tests for XCTest

The `corelibs-xctest` preset is one that swift-corelibs-xctest
contributors could use locally to quickly ensure that XCTest builds and its
tests pass. The libdispatch tests add a significant amount of time to
this preset, so remove them.

Author: modocache

utils/build-presets.ini

@@ -296,6 +296,7 @@ dash-dash
 # being run.
 skip-test-cmark
 skip-test-swift
+skip-test-libdispatch
 skip-test-foundation
 
 # This preset may be used by swift-corelibs-xctest contributors when building