test/system: Optimize the 'create' tests

The system tests can be very I/O intensive, because many of them copy
OCI images from the test suite's image cache directory to its local
container/storage store, create containers, and then delete everything
to run the next test with a clean slate.  This makes them slow.

In the case of these two particular tests, toolbox(1) is supposed to
validate the command line options before trying to find the image.  So,
there's no need to copy the image from the test suite's image cache
directory to its local container/storage store.

If the toolbox(1) code breaks, then either it won't throw the expected
error or it will download the image before validating the command line
options.  The first possibility will be easily detected.  The other
could have been harder to notice, but, fortunately, commit
55c0e63 added variants of these tests without the --assumeyes
option and there are other tests to ensure that images cannot be
downloaded without that option.  So, any unexpected attempts to download
the image will be caught by those variants of these tests.

Fallout from 32b147b

#1595

test/system/101-create.bats

@@ -780,8 +780,6 @@ teardown() {
 }
 
 @test "create: Try using both --distro and --image (using --assumeyes)" {
-  pull_distro_image fedora 34
-
   run "$TOOLBX" --assumeyes create --distro fedora --image fedora-toolbox:34
 
   assert_failure
@@ -802,8 +800,6 @@ teardown() {
 }
 
 @test "create: Try using both --image and --release (using --assumeyes)" {
-  pull_distro_image fedora 34
-
   run "$TOOLBX" --assumeyes create --image fedora-toolbox:34 --release 34
 
   assert_failure