test: unify quotation around image names

test/TEST-03-JOBS/test.sh

@@ -2,6 +2,7 @@
 set -e
 TEST_DESCRIPTION="Job-related tests"
 TEST_NO_QEMU=1
+IMAGE_NAME="default"
 
 . $TEST_BASE_DIR/test-functions
 

test/TEST-13-NSPAWN-SMOKE/test.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -e
 TEST_DESCRIPTION="systemd-nspawn smoke test"
-IMAGE_NAME=nspawn
+IMAGE_NAME="nspawn"
 TEST_NO_NSPAWN=1
 
 . $TEST_BASE_DIR/test-functions

test/TEST-14-MACHINE-ID/test.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -e
 TEST_DESCRIPTION="/etc/machine-id testing"
-IMAGE_NAME=badid
+IMAGE_NAME="badid"
 TEST_NO_NSPAWN=1
 
 . $TEST_BASE_DIR/test-functions

test/TEST-21-SYSUSERS/test.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 set -e
 TEST_DESCRIPTION="Sysuser-related tests"
-IMAGE_NAME=sysusers
+IMAGE_NAME="sysusers"
 . $TEST_BASE_DIR/test-functions
 
 test_setup() {