[swift 4] improve test suite

[4brunu]

The Swift4 test suite is failing because it contains build errors.
This PR tries that and #4560.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

@jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @d-date (2018/03)

[wing328]

Tested locally and the result looks good.

As discussed with @4brunu he wants to add more tests. I'll put the merge on hold.

[4brunu]

I also made Swift Package Manager compile every generated class to make sure the generated code is valid.
To run all the tests just run the following commands

(cd samples/client/petstore/swift4 && sh swift4_test_all.sh)
(cd samples/client/test/swift4 && sh swift4_test_all.sh)

It would be nice to run this on the CI with every PR.

I disabled the test with the sample objcCompatible because I couldn't make it to compile.

[wing328]

I disabled the test with the sample objcCompatible because I couldn't make it to compile.

Don't worry. We can sort it out later.