Fix krb5 external test failure

The krb5 test requires the legacy module to be loaded in order to work.
It also seems to be senstive to using relative paths, so we use absolute
ones instead.

[extended tests]

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10992)

test/recipes/95-test_external_krb5.t

diff --git a/test/recipes/95-test_external_krb5.t b/test/recipes/95-test_external_krb5.t
index 0f2be87..889626a 100644
--- a/test/recipes/95-test_external_krb5.t
+++ b/test/recipes/95-test_external_krb5.t
@@ -10,6 +10,7 @@
 use OpenSSL::Test;
 use OpenSSL::Test::Utils;
 use OpenSSL::Test qw/:DEFAULT data_file srctop_file bldtop_dir/;
+use Cwd qw(abs_path);
 
 setup("test_external_krb5");
 
@@ -20,6 +21,7 @@
 
 plan tests => 1;
 
-$ENV{OPENSSL_CONF} = srctop_file("test", "default-and-legacy.cnf");
+$ENV{OPENSSL_MODULES} = abs_path($ENV{OPENSSL_MODULES});
+$ENV{OPENSSL_CONF} = abs_path(srctop_file("test", "default-and-legacy.cnf"));
 
 ok(run(cmd([data_file("krb5.sh")])), "running krb5 tests");