let Rpc client take a deltachat-rpc-server path and write a test using it

deltachat-rpc-client/tests/test_rpc_virtual.py

@@ -0,0 +1,16 @@
+import subprocess
+import sys
+
+from deltachat_rpc_client import DeltaChat, Rpc
+
+
+def test_install_venv_and_use_other_core(tmp_path):
+    venv = tmp_path.joinpath("venv1")
+    python = sys.executable
+    subprocess.check_call([python, "-m", "venv", venv])
+    subprocess.check_call([venv / "bin" / "pip", "install", "deltachat-rpc-server==2.20.0"])
+    rpc = Rpc(accounts_dir=tmp_path.joinpath("accounts"), rpc_server_path=venv.joinpath("bin", "deltachat-rpc-server"))
+
+    with rpc:
+        dc = DeltaChat(rpc)
+        assert dc.rpc.get_system_info()["deltachat_core_version"] == "v2.20.0"