gitignore: ignore the remaining test-helper binaries

t_hashtable_overflow, t_iwildmatch, t_clean_fname, and t_safe_arg were
built by the suite but missing from .gitignore, so a stray git add -A
sweeps them into a commit (as happened during this rebase round).  List
them alongside the other t_* harnesses.

.gitignore

@@ -48,6 +48,10 @@ aclocal.m4
 /t_rename_secure
 /t_secure_relpath
 /t_symlink_secure
+/t_hashtable_overflow
+/t_iwildmatch
+/t_clean_fname
+/t_safe_arg
 /simdtest
 /wildtest
 /getfsdev