I got stuck in Test Mode. How do I get out? [iOS only]

A hard reboot is the easiest way to fix this.

From Apple: 

• On an iPad Pro 11-inch or iPad Pro 12.9-inch:
  • Press and quickly release the Volume Up button.
  • Press and quickly release the Volume Down button.
  • Then press and hold the Power button until the device restarts.
• On an iPhone 8 or later:
  • Press and quickly release the Volume Up button.
  • Press and quickly release the Volume Down button.
  • Then press and hold the Side button until you see the Apple logo.
• On an iPhone 7 or iPhone 7 Plus:
  • Press and hold both the Side and Volume Down buttons for at least 10 seconds, until you see the Apple logo.
• On an iPhone 6s and earlier, iPad, or iPod touch:
  • Press and hold both the Home and the Top (or Side) buttons for at least 10 seconds, until you see the Apple logo.

Link: https://support.apple.com/en-us/HT201412