test: Drop /usr overlay workaround

/usr is not erofs anymore in the mkosi images so let's drop the
workaround where we mount a writable tmpfs on top of /usr.

test/units/testsuite-15.sh

@@ -8,10 +8,6 @@ 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
-
 clear_unit() {
     local unit_name="${1:?}"
     local base suffix

test/units/testsuite-26.sh

@@ -12,16 +12,10 @@ at_exit() {
         rm -fvr "/usr/lib/systemd/system/$UNIT_NAME" "/etc/systemd/system/$UNIT_NAME.d" "+4"
     fi
 
-    maybe_umount_usr_overlay
-
     rm -f /etc/init.d/issue-24990
     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
-
 # Create a simple unit file for testing
 # Note: the service file is created under /usr on purpose to test
 #       the 'revert' verb as well

test/units/testsuite-60.sh

@@ -6,10 +6,6 @@ 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
-
 teardown_test_dependencies() (
     set +eux
 

test/units/testsuite-73.sh

@@ -9,10 +9,6 @@ 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
-
 enable_debug() {
     mkdir -p /run/systemd/system/systemd-localed.service.d
     cat >>/run/systemd/system/systemd-localed.service.d/override.conf <<EOF

test/units/util.sh

@@ -226,22 +226,6 @@ kernel_supports_lsm() {
     return 1
 }
 
-MOUNTED_USR_OVERLAY=false
-
-maybe_mount_usr_overlay() {
-    if [[ ! -w /usr ]]; then
-        mkdir -p /tmp/usr-overlay/{upperdir,workdir}
-        mount -t overlay -o lowerdir=/usr,upperdir=/tmp/usr-overlay/upperdir,workdir=/tmp/usr-overlay/workdir overlay /usr
-        MOUNTED_USR_OVERLAY=true
-    fi
-}
-
-maybe_umount_usr_overlay() {
-    if "$MOUNTED_USR_OVERLAY"; then
-        umount -l /usr
-    fi
-}
-
 install_extension_images() {
         local os_release
         os_release="$(test -e /etc/os-release && echo /etc/os-release || echo /usr/lib/os-release)"