runtests: fix bundle detection by not looking for units

`units` may be not be built in certain cases when using autotools.

Fixes:
8516da2b-b351-40b2-bf13-7c4ab4bcdd55/jobs/127197

Bug: https://github.com/curl/curl/pull/16750#issuecomment-2738041943
Follow-up to a9b7cbf34f #16750

Closes #16770

tests/runtests.pl

@@ -2514,9 +2514,9 @@ EOHELP
     shift @ARGV;
 }
 
-# Detect a test bundle build
+# Detect a test bundle build.
+# Do not look for 'units' because not all configurations build it.
 if(-e $LIBDIR . "libtests" . exe_ext('TOOL') &&
-   -e $UNITDIR . "units" . exe_ext('TOOL') &&
    -e $SRVDIR . "servers" . exe_ext('SRV')) {
     # use test bundles
     $bundle=1;