use python API for tests

Before:
real	0m4.807s
user	0m3.817s
sys	0m0.920s

After:
real	0m1.236s
user	0m1.011s
sys	0m0.222s

Multiply by number of testcases we didn't implement yet and by
python versions we want to test on.

Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>

test.py

@@ -7,6 +7,8 @@ import textwrap
 
 import pytest
 
+from rust2rpm import Metadata
+
 DUMMY_LIB = """
 pub fn say_hello() {
     println!("Hello, World!");
@@ -36,12 +38,6 @@ def cargo_toml(request):
     return make_cargo_toml
 
 
-def run(*params):
-    cmd = [sys.executable, DEPGEN, *params]
-    out = subprocess.check_output(cmd, universal_newlines=True)
-    return out.split("\n")[:-1]
-
-
 @pytest.mark.parametrize("toml, provides, requires, conflicts", [
 
     # Basic provides
@@ -278,6 +274,7 @@ def run(*params):
 
 ])
 def test_depgen(toml, provides, requires, conflicts, cargo_toml):
-    assert run("--provides", cargo_toml(toml)) == provides
-    assert run("--requires", cargo_toml(toml)) == requires
-    assert run("--conflicts", cargo_toml(toml)) == conflicts
+    md = Metadata.from_file(cargo_toml(toml))
+    assert [str(x) for x in md.provides] == provides
+    assert [str(x) for x in md.requires] == requires
+    assert [str(x) for x in md.conflicts] == conflicts