Hmm, in this case test dependencies becomes hard dependencies for end user. So, for this user there is no difference between PREREQ_PM and BUILD_REQUIRES, he should install both (if he used cpan shell).
It seems all that I can is to ask user:
ExtUtils::MakeMaker::prompt("Do you want testing support for this module?\nThis requires additional modules to be installed: XX::YY, YY::ZZ", "yes");
However it will be strange question when notest pragma specified in the cpan shell. But it seems I can't determine from Makefile.PL is it specified, I didn't found how.

In reply to Re^4: How to specify tests dependencies with Makefile.PL? by OlegG
in thread How to specify tests dependencies with Makefile.PL? by OlegG

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