add test to guard against #984 regression

test/function/preserves-default-exports-used-locally/_config.js

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+module.exports = {
+	description: 'preserves default exports that are only used locally (#984)'
+};

test/function/preserves-default-exports-used-locally/foo.js

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+export default function foo () {
+	global.foo = 1;
+}
+
+foo();

test/function/preserves-default-exports-used-locally/main.js

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+import './foo';
+assert.equal( global.foo, 1 );