That syntax is valid for interpolating a shell variable inside the quotes. I don’t know Rspec, but I assume from the name of the variable that it can potentially hold files plural, in which case the quotes would be necessary (and I assume it would break without the quotes in that case).
I tested this with a simple shell command:
export HELLO='1 2 3' ; echo "${HELLO}"
1 2 3
What value does $TEST_FILES have in this case? I wonder if it has its own quotes?