efivarfs: Fix fs_test.go to end with mutable file
efivarfs: Fix fs_test.go to end with mutable file

f12209a invoked restore to fix a tmpdir removal problem. However,
restore returns the file to an immutable state, not a mutable one. As
such, the file was left in an immutable (and hence unremovable) state at
the end of the test.

The previous code was likely passing tests because the erroneous code
was being skipped when the immutable flag couldn't be set on the test
file. I've tested my changes as root and confirmed that they work and do
not skip any parts of this test.

The following changes are made to fs_test.go:

* Remove the restore == nil check, since makeMutable only returns nil
  restore on error
* To properly test makeMutable:
  * Set the file's immutable bit before calling makeMutable
  * Observe that the file's immutable bit is now unset
  * End the test without calling restore to ensure that the file can be
    cleaned up

Signed-off-by: Conrad Parker <3a93e291600975c62c4f06626c22457b261beb8b@google.com>

u-root/u-root,u-root/u-root,u-root/u-root