Revert "Disable validation-test/Python/build_swift.swift" (#29481)

This reverts commit df2e553.  Repair
the preset rather than disabling the tests.

Author: compnerd

utils/build-presets.ini

@@ -2263,7 +2263,6 @@ install-lldb
 install-llbuild
 install-swiftpm
 install-swiftsyntax
-skip-install-swiftsyntax-module
 install-skstresstester
 install-swiftevolve
 install-playgroundsupport

validation-test/Python/build_swift.swift

@@ -0,0 +1,10 @@
+// Continuous integration for the OS X Platform also runs the tests in the
+// iPhone, Apple TV and Apple Watch simulators. We only need to run the
+// build_swift module unit-tests once per OSX Platform test run, rather than
+// once for each supported Apple device.
+
+// UNSUPPORTED: OS=ios
+// UNSUPPORTED: OS=tvos
+// UNSUPPORTED: OS=watchos
+
+// RUN: %{python} %utils/build_swift/run_tests.py