`copilot-c99`: Change return type of `main` generated for tests. Refs #468.

[ivanperez-keera]

Modify the tests for copilot-c99 so that the main has type int, making it implicitly also return 0, as prescribed in the solution proposed for #468.

[ivanperez-keera]

Change Manager: Verified that:

• Solution is implemented:

  • [X] The code proposed compiles and passes all tests. Details: Build log: https://app.travis-ci.com/github/Copilot-Language/copilot/builds/267854133

  • [X] The solution proposed produces the expected result. Details: This bug only manifests in some architectures, so we don't have a way of always reproducing it. Implementation evaluated by visual inspection of the code.

  For what is worth, searching for void main in the search tree now brings up no results, while searching for int main brings up several results:

  $ grep -nIHre 'void main' 
  $ grep -nIHre 'int main' 
  copilot-c99/tests/Test/Copilot/Compile/C99.hs:160:          , "int main () {"
  copilot-c99/tests/Test/Copilot/Compile/C99.hs:696:    , "int main () {"

  That, however, is a fairly weak check, since splitting the type and the function name would make the search pattern not succeed (which is common for example with other coding standards like GNU's).

• [X] Implementation is documented. Details: Internal change; no changes to the documentation are needed.
• [X] Change history is clear.
• [X] Commit messages are clear.
• [X] Changelogs are updated.
• [X] Examples are updated. Details: No updates needed.
• [X] Required version bumps are evaluated. Details: Bump not needed (internal change to tests; does not affect API).