test: document when writable /usr overlays may be needed

test/units/testsuite-15.sh

@@ -8,6 +8,7 @@ set -o pipefail
 # shellcheck source=test/units/util.sh
 . "$(dirname "$0")"/util.sh
 
+# Needed to write units to /usr/lib/systemd/system to test /etc and /run overrides.
 maybe_mount_usr_overlay
 trap 'maybe_umount_usr_overlay' EXIT
 

test/units/testsuite-26.sh

@@ -18,6 +18,7 @@ at_exit() {
     return 0
 }
 
+# Needed for /usr/lib/systemd/system/$UNIT_NAME to test overrides in /etc and /run
 maybe_mount_usr_overlay
 trap at_exit EXIT
 

test/units/testsuite-60.sh

@@ -6,6 +6,7 @@ set -o pipefail
 # shellcheck source=test/units/util.sh
 . "$(dirname "$0")"/util.sh
 
+# Needed to create mount.mytmpfs helper
 maybe_mount_usr_overlay
 trap 'maybe_umount_usr_overlay' EXIT
 

test/units/testsuite-73.sh

@@ -9,6 +9,7 @@ set -o pipefail
 # shellcheck source=test/units/util.sh
 . "$(dirname "$0")"/util.sh
 
+# Needed to generate test locales in /usr/lib
 maybe_mount_usr_overlay
 trap 'maybe_umount_usr_overlay' EXIT