Fix register naming output in readelf test.

Suppress output if no detailed register name is known. Fixes
following testcase output:

 Test file 'test/testfiles_for_readelf/simple_mips_gcc.o.elf'
 .......................FAIL
 ....for option "--debug-dump=frames"
 ....Output #1 is readelf, Output #2 is pyelftools
 @@ Mismatch on line #7:
 >>  dw_cfa_def_cfa: r29 ofs 0<<
 >>  dw_cfa_def_cfa: r29 (none) ofs 0<<