[gdb/testsuite] Handle maint set dwarf synchronous off default

[gdb/testsuite] Handle maint set dwarf synchronous off default

I ran the testsuite with a patch setting dwarf_synchronous to false by
default, and ran into FAILs in test-cases gdb.dwarf2/dw2-inter-cu-error.exp
and gdb.dwarf2/dw2-inter-cu-error-2.exp, because the expected DWARF errors did
not show up as a result of the file command.

Fix this by forcing "maint set dwarf synchronous on".

Add the same in gdb.base/index-cache.exp, where this is also required.

Tested on aarch64-linux.