Add swift-testing to Linux presets too

Author: stmontgomery

utils/build-presets.ini

@@ -839,6 +839,7 @@ mixin-preset=
 llbuild
 swiftpm
 swift-driver
+swift-testing
 xctest
 swiftdocc
 swiftformat
@@ -852,6 +853,7 @@ install-llbuild
 install-swiftpm
 install-swift-driver
 install-swiftsyntax
+install-swift-testing
 install-xctest
 install-prefix=/usr
 install-sourcekit-lsp