TEST-13-NSPAWN: wait for a few seconds after markers found

Otherwise, the scope that the nspawn container belonging to may be
removed before the grandchild process of the machined exits and it may
be SIGKILLed.
```
[  100.829613] systemd-machined[678]: Successfully forked off '(sd-bindmnt)' as PID 2962.
[  100.833366] systemd-nspawn[2953]: Inner child finished, invoking payload.
[  100.836111] (sd-bindmnt)[2962]: Skipping PR_SET_MM, as we don't have privileges.
[  100.836401] (sd-bindmnt)[2962]: Successfully forked off '(sd-bindmnt-inner)' as PID 2964.
[  100.846498] (sd-bindmnt)[2962]: (sd-bindmnt-inner) terminated by signal KILL.
[  100.848846] systemd[1]: machine-TEST\x2d13\x2dNSPAWN.machinectl\x2dbind.7ye.scope: cgroup is empty
[  100.849303] systemd[1]: machine-TEST\x2d13\x2dNSPAWN.machinectl\x2dbind.7ye.scope: Deactivated successfully.
[  100.849317] systemd[1]: machine-TEST\x2d13\x2dNSPAWN.machinectl\x2dbind.7ye.scope: Changed running -> dead
[  100.849752] systemd[1]: machine-TEST\x2d13\x2dNSPAWN.machinectl\x2dbind.7ye.scope: Consumed 91ms CPU time, 1.3M memory peak.
[  100.850399] systemd-machined[678]: (sd-bindmnt) failed with exit status 1.
[  100.850414] systemd-machined[678]: Child failed.
[  100.854574] systemd-machined[678]: Failed to mount /tmp/marker-varlink on /tmp/marker-varlink in the namespace of machine 'TEST-13-NSPAWN.machinectl-bind.7ye': Protocol error
```

Hopefully fixes #38434.

src/basic/filesystem-sets.py

@@ -282,6 +282,11 @@ def generate_fs_type_to_string():
 #include <linux/magic.h>
 #include "filesystems.h"
 
+/* PROJECT_FILE, which is used by log_xyz() thus also used by assert_not_reached(), cannot be used in
+ * generated files, as the build directory may be outside of the source directory. */
+#undef PROJECT_FILE
+#define PROJECT_FILE __FILE__
+
 const char* fs_type_to_string(statfs_f_type_t magic) {
         switch (magic) {""")
 

test/units/TEST-13-NSPAWN.nspawn.sh

@@ -933,7 +933,7 @@ EOF
 
 testcase_machinectl_bind() {
     local service_path service_name root container_name ec
-    local cmd='for i in $(seq 1 20); do if test -f /tmp/marker && test -f /tmp/marker-varlink; then exit 0; fi; sleep .5; done; exit 1;'
+    local cmd="timeout 10 bash -c 'until [[ -f /tmp/marker ]] && [[ -f /tmp/marker-varlink ]]; do sleep .5; done'; sleep 3"
 
     root="$(mktemp -d /var/lib/machines/TEST-13-NSPAWN.machinectl-bind.XXX)"
     create_dummy_container "$root"