test/README: fix advice for testsuite debugging

KERNEL_APPEND="systemd.unit=multi-user.target" is not very useful, because the
machine will still shut down as soon as the tests succeeds or fails. But
INTERACTIVE_DEBUG=1 works great, so let's simplify the instructions and
recommend that.

(cherry picked from commit b13d592)

Author: keszybz

test/README.testsuite

@@ -99,20 +99,20 @@ INTERACTIVE_DEBUG=1
     the test, etc.)
 
 The kernel and initrd can be specified with $KERNEL_BIN and $INITRD. (Fedora's
-or Debian's default kernel path and initrd are used by default)
+or Debian's default kernel path and initrd are used by default.)
 
 A script will try to find your qemu binary. If you want to specify a different
 one with $QEMU_BIN.
 
 Debugging the qemu image
 ========================
 
-If you want to log in the testsuite virtual machine, you can specify additional
-kernel command line parameter with $KERNEL_APPEND and then log in as root.
+If you want to log in the testsuite virtual machine, use INTERACTIVE_DEBUG=1
+and log in as root:
 
-$ sudo make -C test/TEST-01-BASIC KERNEL_APPEND="systemd.unit=multi-user.target" run
+$ sudo make -C test/TEST-01-BASIC INTERACTIVE_DEBUG=1 run
 
-Root password is empty.
+The root password is empty.
 
 Ubuntu CI
 =========