moved test to src/test/unit/math/functions/sign_test.cpp

#### Summary:

Moves test from src/test/math/functions/sign_test.cpp to src/test/unit/math/functions/sign_test.cpp.

#### Intended Effect:

Test will now be run by Jenkins.

#### How to Verify:

Run the moved unit test.

#### Side Effects:

None.

#### Documentation:

Not user facing.

#### Reviewer Suggestions:

Anyone.