test/test-functions: add mkdir to import_initdir

This dir is created by create_empty_image_rootdir, as well as indirectly
by some other functions, but it should be created by import_initdir so
the newly-exported $initdir exists and can be used immediately without
relying on other functions to create it.

test/test-functions

@@ -941,6 +941,7 @@ import_testdir() {
 
 import_initdir() {
     initdir=$TESTDIR/root
+    mkdir -p $initdir
     export initdir
 }