add test for counting pre option and fix it's duplicate

pipenv/cli/options.py

@@ -375,7 +375,6 @@ def install_options(f):
     f = index_option(f)
     f = extra_index_option(f)
     f = requirementstxt_option(f)
-    f = pre_option(f)
     f = selective_upgrade_option(f)
     f = ignore_pipfile_option(f)
     f = editable_option(f)

tests/unit/test_help.py

@@ -9,3 +9,16 @@ def test_help():
         stderr=subprocess.STDOUT, env=os.environ.copy(),
     )
     assert output
+
+
+def test_count_of_description_pre_option():
+    test_command = 'pipenv install --help'
+    test_line = '--pre Allow pre-releases.'
+    out = subprocess.Popen(test_command.split(), stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
+    stdout, _ = out.communicate()
+    lines = stdout.decode().split('\n')
+    count = 0
+    for line in lines:
+        if line.strip().split() == test_line.split():
+            count += 1
+    assert count == 1