Rename variable "addr" to "coredump_var_addr" in gdb.base/coredump-filter.exp

Rename variable "addr" to "coredump_var_addr" in gdb.base/coredump-filter.exp

This commit renames the global array variable "addr" to an unique name
"coredump_var_addr" in the test gdb.base/coredump-filter.exp.  This is
needed because global arrays can have name conflicts between tests.
For example, this specific test was conflicting with dmsym.exp,
causing errors like:

  ERROR: tcl error sourcing ../../../../../binutils-gdb/gdb/testsuite/gdb.base/dmsym.exp.
  ERROR: can't set "addr": variable is array
      while executing
  "set addr "0x\[0-9a-zA-Z\]+""
      (file "../../../../../binutils-gdb/gdb/testsuite/gdb.base/dmsym.exp" line 45)
      invoked from within
  "source ../../../../../binutils-gdb/gdb/testsuite/gdb.base/dmsym.exp"
      ("uplevel" body line 1)
      invoked from within
  "uplevel #0 source ../../../../../binutils-gdb/gdb/testsuite/gdb.base/dmsym.exp"
      invoked from within
  "catch "uplevel #0 source $test_file_name""

This problem was reported by Yao Qi at:

  <https://sourceware.org/ml/gdb-patches/2015-04/msg00373.html>
  Message-Id: <1428666671-12926-1-git-send-email-qiyaoltc@gmail.com>

gdb/testsuite/ChangeLog:
2015-04-13  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/coredump-filter.exp: Rename variable "addr" to
	"coredump_var_addr" to avoid naming conflict with other testcases.