lightningd/test/run-find_my_abspath.c includes ../lightningd.c, which includes
header_versions_gen.h, a generated header file.
lightningd/Makefile correctly declares that lightningd/lightningd.o depends on
header_versions_gen.h, but lightningd/test/Makefile lacks any such declaration
regarding lightningd/test/run-find_my_abspath.c, which leads to build failure:
In file included from lightningd/test/run-find_my_abspath.c:5:
lightningd/test/../lightningd.c:64:10: fatal error: header_versions_gen.h: No such file or directory
64 | #include <header_versions_gen.h>
| ^~~~~~~~~~~~~~~~~~~~~~~
Declare the missing dependency in lightningd/test/Makefile so that Make will
ensure that header_versions_gen.h is generated before it attempts to build
lightningd/test/run-find_my_abspath.o.
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