Mock generator for tests

Created by: jmini

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @OpenAPITools/generator-core-team

Description of the PR

This PR adds a new class that can be used in tests: MockDefaultGenerator This is a subclass of DefaultGenerator that only modify the way files are written. Instead of writting them to the disks, they are kept in memory.

Usage (example in this PR):

        MockDefaultGenerator generator = new MockDefaultGenerator();
        generator.opts(clientOptInput).generate();

Check on generated files:

        Map<String, String> generatedFiles = generator.getFiles();
        Assert.assertEquals(generatedFiles.size(), 35);
        ensureContainsFile(generatedFiles, output, ".gitignore");
        ensureContainsFile(generatedFiles, output, ".openapi-generator-ignore");
        ensureContainsFile(generatedFiles, output, ".openapi-generator/VERSION");
        ensureContainsFile(generatedFiles, output, ".travis.yml");
        ensureContainsFile(generatedFiles, output, "build.gradle");
        ensureContainsFile(generatedFiles, output, "build.sbt");

Check on the content of a generated file:

        String defaultApiConent = generatedFiles.get(defaultApiFilename);
        Assert.assertTrue(defaultApiConent.contains("public class DefaultApi")); 

Check on the parameters of a template-based generated file:

        Optional<WrittenTemplateBasedFile> optional = generator.getTemplateBasedFiles().stream().filter(f -> defaultApiFilename.equals(f.getOutputFilename())).findFirst();
        Assert.assertTrue(optional.isPresent());
        Assert.assertEquals(optional.get().getTemplateData().get("classname"), "DefaultApi");

The only changes made to the API is to create a new protected method writeInputStreamToFile(String, InputStream, String) in DefaultGenerator.