Add JUnit 4 and the Vintage Engine sample

[ielatif]

This commit adds a sample to verify the use of JUnit 4 and the Vintage Engine.
Closes #17070.

[ielatif]

@philwebb @mbhave @wilkinsona JUnit 5 checkstyle rules introduced in #17093 are applied to spring-boot-sample-junit-vintage which cause it to fail.
Any idea on how to disable only JUnit 5 checkstyle rules?

[ielatif]

May be add a suppression for SampleJunitVintageApplicationTests in checkstyle-suppressions.xml?

[wilkinsona]

@ielatif Yes, I think that's the right approach.

[ielatif]

@wilkinsona raised this PR to fix master CI build failure

[snicoll]

@ielatif thanks for the PR. I've polished in two main areas:

1. I think what we want is to show that adding the starter with no further modification allows us to run a Junit 4-based test and the exclusions could potentially hide a problem. I've removed them
2. There is no need to start an embedded web server for such a sample so I rewrote the test to use mock MVC.

[ielatif]

@snicoll

1. I think what we want is to show that adding the starter with no further modification allows us to run a Junit 4-based test and the exclusions could potentially hide a problem. I've removed them

Totaly agree 👍

2. There is no need to start an embedded web server for such a sample so I rewrote the test to use mock MVC.

I was thinking of the sample as a sample application (application running an embedded web server) and not as a smoke test like @philwebb mentioned in a newly raised feature.