docker: Inline "prep_fail" in run script

We don't source common.rc where prep_fail is defined, so spell out the
commands and do what was intended.
Signed-off-by: NFam Zheng <famz@redhat.com>
Message-Id: <20180326090350.30014-1-famz@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/run

@@ -29,7 +29,7 @@ export TEST_DIR=/tmp/qemu-test
 mkdir -p $TEST_DIR/{src,build,install}
 
 # Extract the source tarballs
-tar -C $TEST_DIR/src -xf $BASE/qemu.tar || prep_fail "Failed to untar source"
+tar -C $TEST_DIR/src -xf $BASE/qemu.tar || { echo "Failed to untar source"; exit 2; }
 if test -f $TEST_DIR/src/Makefile; then
     export FEATURES="$FEATURES dtc"
 fi