Fixup Structure of Mock SDKSettings

This better reflects the actual structure of the files as they appear in
the 10.15 and 10.15.4 SDKs respectively.

rdar://69412779

Author: CodaFi

test/Driver/Inputs/MacOSX10.15.4.versioned.sdk/SDKSettings.json

@@ -23,5 +23,7 @@
           "13.2" : "10.15.1",
           "13.4" : "10.15.4"
       }
-  }
+  },
+  "DefaultDeploymentTarget" : "11.0",
+  "MaximumDeploymentTarget" : "11.0.99"
 }

test/Driver/Inputs/MacOSX10.15.versioned.sdk/SDKSettings.json

@@ -9,5 +9,7 @@
           "13.1" : "10.15",
           "13.2" : "10.15.1"
       }
-  }
+  },
+  "DefaultDeploymentTarget" : "10.15",
+  "MaximumDeploymentTarget" : "10.15.99"
 }