Added Jest snapshot test files as generated src (#3572)

lib/linguist/generated.rb

@@ -81,7 +81,8 @@ module Linguist
       generated_grammarkit? ||
       generated_roxygen2? ||
       generated_jison? ||
-      generated_yarn_lock?
+      generated_yarn_lock? ||
+      generated_jest_snapshot?
     end
 
     # Internal: Is the blob an Xcode file?
@@ -489,5 +490,15 @@ module Linguist
       return false unless lines.count > 0
       return lines[0].include?("# THIS IS AN AUTOGENERATED FILE")
     end
+
+    # Internal: Is this a Jest [snapshot test](https://facebook.github.io/jest/docs/snapshot-testing.html)
+    # file?
+    #
+    # Return true or false.
+    def generated_jest_snapshot?
+      return false unless extname == '.snap'
+      return false unless lines.count > 0
+      return lines[0].start_with?("// Jest Snapshot")
+    end
   end
 end