Like we want to separate libgit2 and utility source code, we want to
separate libgit2 and utility tests.  Start by moving all the tests into
libgit2.

The configuration change will override the environment variable, so the
environment variable check needs to come first.

Signed-off-by: Mathieu Parent <math.parent@gmail.com>

Update the proxy detection for a remote.

1. Honor `http.<url>.proxy` syntax for a remote's direct URL and
   parent URLs.
2. Honor an empty configuration URL to override a proxy configuration.

Add tests to ensure that configuration specificity is honored.