add test for prepare_env

tests.py

 import os
 import pytest
 import importlib
+import subprocess
 mod = importlib.import_module("install-mpsd-software-environment")
 
+
 def test_os_chdir():
     # create a temporary directory for testing
     temp_dir = "/tmp/test_os_chdir"
     os.mkdir(temp_dir)
-    
+
     # initial current working directory
     initial_cwd = os.getcwd()
 
@@ -20,3 +22,24 @@ def test_os_chdir():
 
     # clean up the temporary directory
     os.rmdir(temp_dir)
+
+
+def test_prepare_env():
+    # simulate running ./install-software-environment.py --release dev-23a --target-directory /tmp/test_prepare_env
+    mod.prepare_env("dev-23a", "/tmp/test_prepare_env", False)
+    # check if the directory exists
+    assert os.path.exists("/tmp/test_prepare_env")
+    # check for spack-environments directory
+    assert os.listdir("/tmp/test_prepare_env") == ["spack-environments"]
+    # check if the git branch is correctl checked out
+    assert (
+        subprocess.run(
+            "cd /tmp/test_prepare_env/spack-environments && git branch",
+            shell=True,
+            capture_output=True,
+        ).stdout.decode("utf-8")
+        == "* dev-23a\n"
+    )
+    # make sure running the command again raises ValueError
+    with pytest.raises(ValueError):
+        mod.prepare_env("dev-23a", "/tmp/test_prepare_env", False)