[gdb/testsuite] Fix duplicates in gdb.base/watchpoint-unaligned.exp

When running test-case gdb.base/watchpoint-unaligned.exp on ppc64le-linux, we
get:
...
XFAIL: $exp: rwatch data.u.size1[3] (PRMS breakpoints/23131)
XFAIL: $exp: rwatch data.u.size1[4] (PRMS breakpoints/23131)
  ...
UNTESTED: $exp: wpcount(4)
XFAIL: $exp: rwatch data.u.size1[3] (PRMS breakpoints/23131)
DUPLICATE: $exp: rwatch data.u.size1[3] (PRMS breakpoints/23131)
XFAIL: $exp: rwatch data.u.size1[4] (PRMS breakpoints/23131)
DUPLICATE: $exp: rwatch data.u.size1[4] (PRMS breakpoints/23131)
  ...
UNTESTED: $exp: wpcount(7)
...

Fix this by using foreach_with_prefix.

Tested on ppc64le-linux.