usually some variation of "Guardy", which is my common user name all around the web (which... probably says an uncomfortable amount of things about how I spend my spare time, now that I think of it) and/or "test test test".
I have used "And your heart is Amelia, dying to fly" occasionally - it's a line from Mary Black's song No Frontiers. Mostly because I (used to? Eh, whom am I kidding) have the worst history crush on Amelia Earhart. Plus, it's just such a nice line.
I've also used other lyrics, now and then - I really love my music.
If I need a longer piece of text, I often use
"When a golden girl can win
Prayer from the lips of sin
When the barren almond bears
And a little child gives away its tears
Then shall all the house be still
And peace come to Canterville"
from Oscar Wilde's "Canterville Ghost", because it's still one of my favorite pieces of fiction.