Ensure tests find the right libPackageDescription

Author: mxcl

Support/swiftpm.xcodeproj/xcshareddata/xcschemes/swiftpm.xcscheme

@@ -139,6 +139,11 @@
             value = "$(BUILT_PRODUCTS_DIR)/swift-build-tool"
             isEnabled = "YES">
          </EnvironmentVariable>
+         <EnvironmentVariable
+            key = "SPM_INSTALL_PATH"
+            value = "$(PROJECT_DIR)/../.build"
+            isEnabled = "YES">
+         </EnvironmentVariable>
          <EnvironmentVariable
             key = "SWIFTC"
             value = "$(SWIFT_EXEC)"