[ScanDependency] Allow continue searching for testable module

When swift dependency scanner first finds a binary module for a testable
import, verify if the module is built for enable-testing or not. If not,
keeps searching in case there is a second testable binary module in the
search path.

Previously, the first binary module will always be accepted by scanner
and rely on the importer to provide a good diagnostics. Now the scanner
will emit a warning before continue searching, so user understands why
the binary in the search path is not taken.

lib/Serialization/SerializedModuleLoader.cpp

@@ -454,6 +454,12 @@ SerializedModuleLoaderBase::scanModuleFile(Twine modulePath, bool isFramework,
                            modulePath.str());
       return std::make_error_code(std::errc::no_such_file_or_directory);
     }
+
+    if (isTestableImport && !loadedModuleFile->isTestable()) {
+      Ctx.Diags.diagnose(SourceLoc(), diag::skip_module_not_testable,
+                         modulePath.str());
+      return std::make_error_code(std::errc::no_such_file_or_directory);
+    }
   }
 
   // Some transitive dependencies of binary modules are not required to be