test/asn1_internal_test.c: silence the new check for the ASN1 method table

In 38eca7fe

 a new check for the pem_str member of the entries of the
ASN1 method table was introduced. Because the test condition was split
into two TEST_true(...) conditions, the test outputs error diagnostics
for all entries which have pem_str != NULL. This commit joins the two
test conditions into a single condition.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6888)