I did a Google search and did not find anything about this. I assume that one of the plugins you have installed is creating this test file for some reason, which is probably to check if the /wp-content/ folder is writable for other real/legitimate plugin files that will be created in the /wp-content/ folder. The PHP is_writable() function can check both files or a directory to see if it is writable so there would not be any reason to create a temp file to do that. The file naming convention is odd, but # characters are allowed in file naming conventions. To find out which plugin is creating this file you can use the BPS Pro String|Function Finder Pro-Tool to search your /plugins/ folder using these search parameters:
Search String: temp-write-test
Search Path: /path/to/your/plugins/folder (Your Website Root Path is displayed on the page so just add the rest of the folder path to your plugins folder)