WiX: extract Testing Macros into a component (NFC)

Extract the testing macros into their own component group. Each
repository is currently setup into a single component group that we then
enumerate into the install image. This follows suit for the testing
macros.

(cherry picked from commit 0932d6b)

Author: compnerd

platforms/Windows/bld/bld.wxs

@@ -324,6 +324,9 @@
       <Component>
         <File Source="$(TOOLCHAIN_ROOT)\usr\bin\SwiftMacros.dll" />
       </Component>
+    </ComponentGroup>
+
+    <ComponentGroup Id="TestingMacros" Directory="_usr_bin">
       <Component>
         <File Source="$(TOOLCHAIN_ROOT)\usr\bin\TestingMacros.dll" />
       </Component>
@@ -428,6 +431,7 @@
       <ComponentGroupRef Id="swift_driver" />
       <ComponentGroupRef Id="swift_syntax" />
       <ComponentGroupRef Id="SwiftMacros" />
+      <ComponentGroupRef Id="TestingMacros" />
 
       <ComponentGroupRef Id="ClangResources" />