gdb/testsuite: use a more unique name in gdb.mi/mi-breakpoint-multiple-locations.exp

gdb/testsuite: use a more unique name in gdb.mi/mi-breakpoint-multiple-locations.exp

I see failures in this test, due to the function name "add" being too
generic, and unexpected breakpoint locations being found in my
libstdc++, such as (wrapped for readability):

    {
  	number="2.4",enabled="y",addr="0x00007ffff7d67e68",
  	func="(anonymous namespace)::fast_float::bigint::add",
	file="/usr/src/debug/gcc/libstdc++-v3/src/c++17/fast_float/fast_float.h",
	fullname="/usr/src/debug/gcc/libstdc++-v3/src/c++17/fast_float/fast_float.h",
	line="1815", thread-groups=["i1"]
    }

Change the test to use a more unique name.

Change-Id: I91de781be62d246eb41c73eaa410ebdd12633d1d