dev-lang/elixir: add git test dependency, bug #716038

dev-lang/elixir: add git test dependency, bug #716038

Without `git` installed tests fail as:

```
** (ErlangError) Erlang error: :enoent
    (elixir 1.10.2) lib/system.ex:795: System.cmd("git", ["--version"],
    [cd: "/var/tmp/portage/dev-lang/elixir-1.10.2/work/elixir-1.10.2/lib/mix",
    into: "", stderr_to_stdout: true])
    (mix 1.10.2) lib/mix/scm/git.ex:253: Mix.SCM.Git.git!/2
    (mix 1.10.2) lib/mix/scm/git.ex:293: Mix.SCM.Git.git_version/0
    test/test_helper.exs:12: (file)
make: *** [Makefile:113: test_mix] Error 1
```

Reported-by: Agostino Sarubbo
Closes: https://bugs.gentoo.org/716038
Package-Manager: Portage-2.3.96, Repoman-2.3.22
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>