Update repositories.txt (#838)

Author: ladyada

repositories.txt

@@ -432,6 +432,7 @@ https://github.com/adafruit/TinyRTCLib
 https://github.com/adafruit/TinyWireM
 https://github.com/adafruit/TinyXML
 https://github.com/adafruit/WaveHC
+https://github.com/adafruit/Adafruit_TestBed
 https://github.com/adameat/MaximWire
 https://github.com/ademuri/esp-simple-web-dashboard
 https://github.com/ademuri/smart-input-filter