execve: Move KUnit tests to tests/ subdirectory

Move the exec KUnit tests into a separate directory to avoid polluting
the local directory namespace. Additionally update MAINTAINERS for the
new files.

Reviewed-by: David Gow <[e-mail]>
Reviewed-by: SeongJae Park <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

MAINTAINERS

···
 F:	fs/*binfmt_*.c
 F:	fs/Kconfig.binfmt
 F:	fs/exec.c
-F:	fs/exec_test.c
+F:	fs/tests/binfmt_*_kunit.c
+F:	fs/tests/exec_kunit.c
 F:	include/linux/binfmts.h
 F:	include/linux/elf.h
 F:	include/uapi/linux/binfmts.h

fs/binfmt_elf.c

···
 module_exit(exit_elf_binfmt);
 
 #ifdef CONFIG_BINFMT_ELF_KUNIT_TEST
-#include "binfmt_elf_test.c"
+#include "tests/binfmt_elf_kunit.c"
 #endif

fs/exec.c

···
 #endif /* CONFIG_SYSCTL */
 
 #ifdef CONFIG_EXEC_KUNIT_TEST
-#include "exec_test.c"
+#include "tests/exec_kunit.c"
 #endif