Adjust special handling of existing read-only output files

Instead of duplicating the permissions on the temp file, the permissions
are duplicated on the output file directly. If the output file exists as
read-only, it is made writeable first.

There are some environments which don't allow the mv to work if the file
itself is read-only.

test/test_unit_template_default.py

@@ -127,6 +127,12 @@ def test_template_default(runner, yadm, tmpdir):
     input_file.chmod(FILE_MODE)
     output_file = tmpdir.join('output')
 
+    # ensure overwrite works when file exists as read-only (there is some
+    # special processing when this is encountered because some environments do
+    # not properly overwrite read-only files)
+    output_file.write('existing')
+    output_file.chmod(0o400)
+
     script = f"""
         YADM_TEST=1 source {yadm}
         set_awk

test/test_unit_template_esh.py

@@ -86,6 +86,12 @@ def test_template_esh(runner, yadm, tmpdir):
     input_file.chmod(FILE_MODE)
     output_file = tmpdir.join('output')
 
+    # ensure overwrite works when file exists as read-only (there is some
+    # special processing when this is encountered because some environments do
+    # not properly overwrite read-only files)
+    output_file.write('existing')
+    output_file.chmod(0o400)
+
     script = f"""
         YADM_TEST=1 source {yadm}
         local_class="{LOCAL_CLASS}"

test/test_unit_template_j2.py

@@ -88,6 +88,12 @@ def test_template_j2(runner, yadm, tmpdir, processor):
     input_file.chmod(FILE_MODE)
     output_file = tmpdir.join('output')
 
+    # ensure overwrite works when file exists as read-only (there is some
+    # special processing when this is encountered because some environments do
+    # not properly overwrite read-only files)
+    output_file.write('existing')
+    output_file.chmod(0o400)
+
     script = f"""
         YADM_TEST=1 source {yadm}
         local_class="{LOCAL_CLASS}"