gdb: testsuite: fix failed testcases in gdb.base/gdb-caching-proc.exp

gdb: testsuite: fix failed testcases in gdb.base/gdb-caching-proc.exp

When execute the following command:

  make check-gdb TESTS="gdb.base/gdb-caching-proc.exp"

we can see there exist some failed testcases:

  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 0: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 1: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 2: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 3: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 4: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 5: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 6: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 7: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 8: can spawn for attach (got interactive prompt)
  FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 9: can spawn for attach (got interactive prompt)

here are the detailed messages in gdb/testsuite/gdb.log:

  attach 873776
  A program is being debugged already.  Kill it? (y or n) n
  Not killed.
  (gdb) FAIL: gdb.base/gdb-caching-proc.exp: can_spawn_for_attach: 0: can spawn for attach (got interactive prompt)

so handle the case "A program is being debugged already.  Kill it" in
can_spawn_for_attach to fix the failed testcases.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>