move all plugin tests out of babel-core and into their appropriate folders

packages/babel-plugin-syntax-trailing-function-commas/test/fixtures/trailing-function-commas/call/actual.js

@@ -0,0 +1,4 @@
+Math.max(1,
+         2,
+         3,
+);

packages/babel-plugin-syntax-trailing-function-commas/test/fixtures/trailing-function-commas/call/expected.js

@@ -0,0 +1 @@
+Math.max(1, 2, 3);