test-cfs: Fix possible test failure after reboot

Remove the file at the beginning of the test, before opening it for
writing, in order to start the test with an empty file system, not only
after flashing the test, but also following every reboot.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>

examples/cfs-coffee/test-cfs.c

@@ -54,6 +54,7 @@ PROCESS_THREAD(cfs_process, ev, data)
     uint16_t filesize = 65000;
 #define CHUNKSIZE 128
 
+    cfs_remove("hej");
     fd = cfs_open("hej", CFS_WRITE);
     if(fd < 0) {
       printf("could not open file for writing, aborting\n");