With --enable-hardcoded-path-in-tests, $(test-program-prefix)
does not redirect to the built glibc, but we need to run
iconv (the program) against the built glibc even with
--enable-hardcoded-path-in-tests, as it is using the ABI
path for the dynamic linker (as an installed program).
Use $(run-program-prefix) instead.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>