What I ended up doing, which worked really well, was creating an email with some text in the body:
This is a test of the email system. This is not a real spam message, but rather one that is trying to purposely get into the spam folder of another account. This is so that I can ensure that my Gmail filters are setup correctly.
... and a subject line such as:
Test Spam Message 1
I always incremented the number at the end of the subject for each message I sent. This way it doesn't look like a duplicate to Gmail and is actually delivered.
Next, on the receiving account, I decided to go ahead and mark the message as spam. I read in the help that it mentions that marking a message as spam is reversible. They even give an example of how a user might accidentally mark a message as spam, then undo it. This is exactly what I am hoping to emulate.
After marking the first message as spam, all the others (which contain the exact same body and only differ in the subject by one letter) are sent to spam as well.
Once I was done with the testing, I marked all the messages as non-spam. Hopefully Gmail treats this as a user accidentally marking a message as spam and doesn't resort to blacklisting the address or doing something worse.
Update: After trying this again, I had to mark a message as spam twice before anything started happening, and then I noticed weird behaviors on the account that was sending the "spam". The account was automatically logged off, and I couldn't log back in for a while. This might be an automated measure Gmail takes when users report other Gmail users as spammers.