Shell: Make sure all tests put their temp dirs in /tmp

Follow-on to #7337. Been seeing other CI test failures that point to
these temp directories, so let's just move all of them to /tmp. I'm sure
someone will write ext2fs stress tests later :^)

Example:

/usr/Tests/Shell/control-structure-as-command.sh
  Core::Socket: Failed to connect() to /tmp/portal/inspectables: ...
  + rm -rf shell-test 2>/dev/null
  + mkdir shell-test
  Error: The action has timed out.

Userland/Shell/Tests/valid.sh

@@ -65,6 +65,7 @@ word_count=(() | wc -w)
 if not test "$(echo well hello friends $word_count)" -eq 3  { fail variable containing pipeline }
 
 # Globs
+rm -fr /tmp/sh-test 2> /dev/null
 mkdir -p /tmp/sh-test
 pushd /tmp/sh-test
     touch (a b c)(d e f)