Add test with conditional shouldFix

This is to verify that the problem can be used to return true or false
conditionally, and that eslint will only apply fixes when true is
returned.

tests/lib/util/source-code-fixer.js

@@ -204,6 +204,13 @@ describe("SourceCodeFixer", () => {
                 assert.equal(result.output, TEST_CODE);
             });
 
+            it("should only apply fixes for which the 'shouldFix' function returns true", () => {
+                const shouldFixSpy = sinon.spy(problem => problem.message === "foo");
+                const result = SourceCodeFixer.applyFixes(sourceCode, [INSERT_AT_START, REPLACE_ID], shouldFixSpy);
+
+                assert.equal(result.output, "var foo = 6 * 7;");
+            });
+
             it("is called without access to internal eslint state", () => {
                 const shouldFixSpy = sinon.spy();