[wasm] Build and install XCTest for Wasm Swift SDK

Install static XCTest library and its Swift module files in the following
directory structure:

```
usr/lib/swift_static/wasi/libXCTest.a
usr/lib/swift_static/wasi/XCTest.swiftmodule
```

Author: kateinoigakukun

utils/swift_build_support/swift_build_support/products/wasmswiftsdk.py

@@ -193,6 +193,26 @@ def _build_swift_testing(self, swift_host_triple, has_pthread, wasi_sysroot):
             shell.call([self.toolchain.cmake, '--install', '.', '--prefix', '/usr'],
                        env={'DESTDIR': dest_dir})
 
+    def _build_xctest(self, swift_host_triple, has_pthread, wasi_sysroot):
+        xctest = CMakeProduct(
+            args=self.args,
+            toolchain=self.toolchain,
+            source_dir=os.path.join(
+                os.path.dirname(self.source_dir), 'swift-corelibs-xctest'),
+            build_dir=os.path.join(self.build_dir, 'xctest', swift_host_triple))
+        self._append_platform_cmake_options(
+            xctest.cmake_options, swift_host_triple, has_pthread, wasi_sysroot,
+            extra_swift_flags=[])
+        xctest.cmake_options.define('BUILD_SHARED_LIBS', 'FALSE')
+
+        xctest.build_with_cmake([], self.args.build_variant, [],
+                                prefer_native_toolchain=True,
+                                ignore_extra_cmake_options=True)
+        dest_dir = self._target_package_path(swift_host_triple)
+        with shell.pushd(xctest.build_dir):
+            shell.call([self.toolchain.cmake, '--install', '.', '--prefix', '/usr'],
+                       env={'DESTDIR': dest_dir})
+
     def _build_target_package(self, swift_host_triple, has_pthread,
                               stdlib_build_path, llvm_runtime_libs_build_path,
                               wasi_sysroot):
@@ -218,6 +238,7 @@ def _build_target_package(self, swift_host_triple, has_pthread,
         self._build_foundation(swift_host_triple, has_pthread, wasi_sysroot)
         # Build swift-testing
         self._build_swift_testing(swift_host_triple, has_pthread, wasi_sysroot)
+        self._build_xctest(swift_host_triple, has_pthread, wasi_sysroot)
 
         return dest_dir