Make test messages in gdb.mi/mi-var-display.exp unique

Make test messages in gdb.mi/mi-var-display.exp unique

When I fix a bug in gdb.mi/mi-var-display.exp, I find its test
messages aren't unique, which makes some confusions for me.

$ cat testsuite/gdb.sum | grep "PASS" | sort | uniq -c | sort -n
...
 2 PASS: gdb.mi/mi-var-display.exp: set format variable bar
 2 PASS: gdb.mi/mi-var-display.exp: set format variable foo
 2 PASS: gdb.mi/mi-var-display.exp: set format variable weird.func_ptr
 2 PASS: gdb.mi/mi-var-display.exp: set format variable weird.func_ptr_ptr
 2 PASS: gdb.mi/mi-var-display.exp: show format variable foo
 3 PASS: gdb.mi/mi-var-display.exp: eval variable foo

This patch is to make test messages in mi-var-display.exp unique.

gdb/testsuite:

2014-08-09  Yao Qi  <yao@codesourcery.com>

	PR testsuite/13443
	* gdb.mi/mi-var-display.exp: Make test messages unique.