[Downloader] Install SHARED_LIBRARY requirements (#2384)

SHARED_LIBRARY Installable types did not have the requirements as
defined in info.json automatically installed. This change updates the
installation of libraries to also install their requirements.

Resolves #2381

redbot/pytest/downloader.py

@@ -14,7 +14,9 @@ __all__ = [
     "repo_norun",
     "bot_repo",
     "INFO_JSON",
+    "LIBRARY_INFO_JSON",
     "installable",
+    "library_installable",
     "fake_run_noprint",
 ]
 
@@ -92,6 +94,19 @@ INFO_JSON = {
     "type": "COG",
 }
 
+LIBRARY_INFO_JSON = {
+    "author": ("seputaes",),
+    "bot_version": (3, 0, 0),
+    "description": "A long library description",
+    "hidden": False,  # libraries are always hidden, this tests it will be flipped
+    "install_msg": "A library install message",
+    "required_cogs": {},
+    "requirements": ("tabulate"),
+    "short": "A short library description",
+    "tags": ("libtag1", "libtag2"),
+    "type": "SHARED_LIBRARY",
+}
+
 
 @pytest.fixture
 def installable(tmpdir):
@@ -101,3 +116,13 @@ def installable(tmpdir):
 
     cog_info = Installable(Path(str(cog_path)))
     return cog_info
+
+
+@pytest.fixture
+def library_installable(tmpdir):
+    lib_path = tmpdir.mkdir("test_repo").mkdir("test_lib")
+    info_path = lib_path.join("info.json")
+    info_path.write_text(json.dumps(LIBRARY_INFO_JSON), "utf-8")
+
+    cog_info = Installable(Path(str(lib_path)))
+    return cog_info