Some tests make a hard-coded call to the libtest binary in the precheck
step. With bundle builds the binary changed name and calling convention.
Before this patch these tests failed the pre-check and did not run for
the 5 affected tests: 518, 537, 678, 1517, 1960
Fixing, e.g.:
```
test 1517 SKIPPED: precheck command error
```
https://github.com/curl/curl/actions/runs/16611990422/job/46996698437?pr=18039#step:13:4832
It also fixes builds with a custom `CURL_DIRSUFFIX` set.
Follow-up to 2c27a67daa#17590
Follow-up to 71cf0d1fca#14772Closes#18099
Otherwise, it might find the binary in .libs which can cause it to use
the system libcurl which can fail. This error is only visible by
noticing that the test is skipped.
Follow-up to e4dfe6fc
Ref: #10651
When returned from the CURLOPT_SOCKOPTFUNCTION, like when we have a
custom socket connected in the app, passed in to libcurl.
Verifies the fix in #10648Closes#10651
