[gdb/testsuite] Make gdb.dwarf2/dwzbuildid.exp more robust

[gdb/testsuite] Make gdb.dwarf2/dwzbuildid.exp more robust

I ran test-case gdb.dwarf2/dwzbuildid.exp with target board cc-with-gdb-index,
and noticed that compilation failure for one exec prohibited testing of all
execs.

Fix this by restructuring the test-case, such that we have:
...
PASS: gdb.dwarf2/dwzbuildid.exp: testname=ok: set debug-file-directory
PASS: gdb.dwarf2/dwzbuildid.exp: testname=ok: print the_int
UNSUPPORTED: gdb.dwarf2/dwzbuildid.exp: testname=mismatch: compilation failed
UNSUPPORTED: gdb.dwarf2/dwzbuildid.exp: testname=fallback: compilation failed
...

Tested on x86_64-linux.