Eiffel has design by contract build in, as does D.
I'm not sure this is exactly what you are asking about, but Rust does support testing right in the language. You can add #[test] before a function to make it a test function (tests and regular functions can live together in the same file).
Assertions in Java exist. Not really tests per se, but the closest thing I could think of.
Look up "model checking" and see how that compares to what you're thinking about. How would you specify tests (or rather, correct program behavior) in such a language?
The languages Alloy and TLA+ come to mind. (So does Coq, which is at the other extreme of actually proving your program correct.)
Clojure's most recent builds include "spec". http://clojure.org/guides/spec
> perhaps does not provide full proof/correctness/contract support but does come with testing right out of the box, either in its implementation or philosophy?
Python has doctests.
Not sure if thats what you mean?
Like Rust: https://doc.rust-lang.org/book/testing.html ?
Yeah, Pyret!
Rust has cargo test and cargo benchmark. Its basically a unit testing framework and benchmarking framework that is provided "batteries included" with the language distribution.
Its dirt simple to add testing to your project, just create a directory called tests in top level of your project and place rust source files inside them. It then looks for functions inside those files that are annotated as being tests. Similar setup for benchmarks.
Rust also includes generating documentation from comments inside the source code just like doxygen. And it supports embedded examples, which will be checked if those example programs compile at all. It really helps with prevent documentation from going out of date.
All of this is provided from the start when you install rust.
https://en.wikipedia.org/wiki/Design_by_contract
https://coq.inria.fr/about-coq
http://www.adacore.com/adaanswers/about/ada-comparison-chart
http://www.adacore.com/gnatpro/toolsuite/utilities/
https://github.com/lampepfl/dotty
http://www.scalatest.org/getting_started_with_feature_spec
http://etorreborre.github.io/specs2/
http://www.gebish.org/
http://spockframework.github.io/spock/docs/1.1-rc-2/introduc...