Why can't I see all of my test emails in Gmail?

If you cannot see more than one of your test emails but you know that you have sent more than one, this is because Gmail uses Conversational Threading, an action which groups emails from the same sender or the same "conversation" together.
 
If multiple test emails have been sent to your Gmail account, you may see a number next to the From name. When the From name or subject line is clicked, each test email will be listed, however, only the one at the top will display. The others will only say "show quoted text" because the content of the "conversation" has not changed (there were not significant differences between each test).
 
Use the Expand option to the right, or click the top-most email, to view the contents. To view the contents of subsequent tests, click the "show quoted test" link.
 
To avoid having your emails bundled together in this fashion, delete your first test email before sending another one. This way only one test email will be in the Gmail in-box at a time.