libtests: make test 1503,1504,1505 use the 1502 binary

libtests: make test 1503,1504,1505 use the 1502 binary

Adjust the differences at runtime instead of build-time, to avoid
extra buillds.

Set the `CURL_TESTNUM` env variable to pass test numbers to tests.

Make libtest/first.c use that env variable to set the `testnum` global
variable to allow tests to differ based on which test that runs it.

Closes #17591