test: Initialise ${TRACE} properly

Unlike ${DEBUG} we don't initialize ${TRACE} to 0 if not set, which cases
failures when testing it later.  That failure acts as though it is false,
however it emits spurious errors in script.log, which can make it harder to
spot real errors.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

test/run

@@ -32,6 +32,9 @@ FAST=${FAST:-1}
 # If set, run passt and pasta with debug options
 DEBUG=${DEBUG:-0}
 
+# If set, run passt and pasta with trace options
+TRACE=${TRACE:-0}
+
 # If set, tell passt and pasta to take packet captures
 PCAP=${PCAP:-0}