[gdb/testsuite] Check compilation result in gdb.dwarf2/dw2-opt-structptr.exp

[gdb/testsuite] Check compilation result in gdb.dwarf2/dw2-opt-structptr.exp

When running test-case gdb.dwarf2/dw2-opt-structptr.exp with target board
unix/-bad, I get:
...
gdb compile failed, gcc: error: unrecognized command line option '-bad'
UNTESTED: gdb.dwarf2/dw2-opt-structptr.exp: dw2-opt-structptr.exp
UNTESTED: gdb.dwarf2/dw2-opt-structptr.exp: failed to compile
ERROR: (dw2-opt-structptr) No such file or directory
UNRESOLVED: gdb.dwarf2/dw2-opt-structptr.exp: console: set print object on
...

Merge the two UNTESTEDs.

Fix the UNRESOLVED by checking result of compilation.

Tested on x86_64-linux.