always use the core serializers when testing in core

This change has already been applied to blead in advance of perl 5.25.4.

Author: karenetheridge

t/min_perl_version.t

@@ -52,6 +52,7 @@ END
 );
 
 # avoid environment variables interfering with our make runs
+delete @ENV{qw(PERL_JSON_BACKEND CPAN_META_JSON_BACKEND PERL_YAML_BACKEND)} if $ENV{PERL_CORE};
 delete @ENV{qw(LIB MAKEFLAGS PERL_CORE)};
 
 my $perl     = which_perl();

t/several_authors.t

@@ -52,6 +52,7 @@ END
 );
 
 # avoid environment variables interfering with our make runs
+delete @ENV{qw(PERL_JSON_BACKEND CPAN_META_JSON_BACKEND PERL_YAML_BACKEND)} if $ENV{PERL_CORE};
 delete @ENV{qw(LIB MAKEFLAGS PERL_CORE)};
 
 my $perl     = which_perl();