tests: Use unique or temporary names for temporary files.

Review every test(!) to ensure that it:

 - Doesn't use a generic name (eg. "test1.img", "test.out") for any
   temporary file it needs.

 - Does instead use a unique name or a temporary name (eg. a name like
   "name-of-the-test.img", or a scratch disk).

 - Does not use 'rm -f' to clean up its temporary files (so we can
   detect errors if the wrong temporary file is created or removed).

This allows tests to be run in parallel, so they don't stomp on each
other's temporary files.

fish/test-edit.sh

@@ -28,13 +28,13 @@ test -d /dev/fd || {
 
 set -e
 
-rm -f test1.img
+rm -f test-edit.img
 
 # The command will be 'echo ... >>/tmp/tmpfile'
 export EDITOR="echo second line of text >>"
 
 output=$(
-./guestfish -N fs -m /dev/sda1 <<EOF
+./guestfish -N test-edit.img=fs -m /dev/sda1 <<EOF
 write /file.txt "this is a test\n"
 chmod 0600 /file.txt
 chown 10 11 /file.txt
@@ -57,4 +57,4 @@ gid: 11" ]; then
     exit 1
 fi
 
-rm -f test1.img
+rm test-edit.img