Test against small (fake) /tmp dir

This is some test cases for:

https://bugzilla.redhat.com/show_bug.cgi?id=1703278

which provides some failing tests for when `validate_free_space` checks
the wrong directory, and helps validate that the fix here:

ManageIQ/manageiq#18745

Does what it is supposed to.

Thanks a bunch to Dan Berger for the assist on this and pointing me in
the direction to use loop back devices.  Turned out to be a much better
solution then what I was considering with vagrant directly.

How it works
------------

This patch effectively does three things:

First, it adds a provision step on VM boot to add a loop back device (or
a file that acts like a file system) to represent a "fake tmp" directory
that is under 2MB in size after file system overhead.  The process for
building this can be found in the "mount_fake_tmp" provision step, but
the file is writable to just like a normal mountable file system, and
since it is so small it won't fit even our ~2MB database dumps.

Of note, something that was require was the `chmod +t`, which adds the
sticky bit on to the dir.  This is essential for the next part which...

Secondly, adds a patch to `tmpdir` which forces `Dir.tmpdir` to return
`/fake_tmp` instead of `/tmp`.  This part we override both the
`@@systmpdir` var and the `ENV['TMPDIR']` values since either one could
be used in determining the `Dir.tmpdir` depending on the value of
`$SAFE`.

Since usually we aren't using `@@systmpdir`, the `chmod +t ...` becomes
important here since it allows the `File::Stat#sticky?` check to pass,
allowing `/fake_tmp` dir to be a vaild option, and not fall back to
`/tmp` instead.

Finally, we inject the patch into some tests to make sure that those
tests still function when the "tmp" directory has a file that is smaller
then the resulting dump/backup.  These new tests fail without the patch
from above, and pass with it.


(transferred from https://gist.github.com/NickLaMuro/8438015363d90a2d2456be650a2b9bc6/bf256dd6a2f385a78cf945e4efea5084f63812e1)

Vagrantfile

@@ -39,6 +39,28 @@ Vagrant.configure("2") do |config|
     miq.vm.synced_folder "../manageiq-appliance_console", "/vagrant/manageiq-appliance_console", rsync_opts
     miq.vm.synced_folder "./",                            "/vagrant/tests",                      rsync_opts
 
+    # Create a small disk we can add to the VM that we can stub in for `/tmp`
+    #
+    # Only make it 3MB in size (smaller than DB dump size after FS overhead)
+    fake_tmp_fstab_entry = [
+      "/dev/loop0".rjust(41),
+      "/fake_tmp".rjust(37),
+      "ext4".ljust(16),
+      "loop".ljust(15),
+      "0".ljust(8),
+      "0"
+    ].join " "
+    miq.vm.provision "mount_fake_tmp", :type => "shell", :inline => <<-MOUNT_DISK
+      dd if=/dev/zero of=/tmp/fake_tmp_fs bs=3M count=1
+      losetup /dev/loop0 /tmp/fake_tmp_fs
+      mkfs.ext4 /dev/loop0
+      mkdir /fake_tmp
+      echo "#{fake_tmp_fstab_entry}" >> /etc/fstab
+      mount /dev/loop0
+      chmod -R 777 /fake_tmp
+      chmod +t /fake_tmp
+    MOUNT_DISK
+
     ###### copy and update ssh key permissions
     miq.vm.provision "ssh_key", :type => "shell", :inline => <<-SSH_KEY
       cp /vagrant/tests/share.id_rsa /home/vagrant/.ssh/