[gdb/testsuite] Drop address from test name in gdb.mi/mi-memory-changed.exp

[gdb/testsuite] Drop address from test name in gdb.mi/mi-memory-changed.exp

I ran the testsuite twice, once with target board unix, and once with target
board unix/-fPIE/-pie, compare the two sum files, and got for test-case
gdb.mi/mi-memory-changed.exp:
...
< PASS: $exp: set var *(unsigned int *) 0x4011b0 = 0xe5894855
---
> PASS: $exp: set var *(unsigned int *) 0x5555555551c3 = 0xe5894855
...

Fix this by dropping the concrete address from the test name:
...
PASS: $exp: set var *(unsigned int *) 0x${main_addr} = ${main_insn}
...

Tested on x86_64-linux.