[gdb/testsuite] Fix mi_clean_restart <absolute filename> in gdb.mi

Fix mi_clean_restart <absolute filename> in the test-cases in gdb.mi.

Tested on x86_64-linux.

Also tested test-case gdb.mi/mi-dprintf.exp with target boards
native-gdbserver and native-extended-gdbserver.

Since test-case gdb.mi/mi-regs.exp requires istarget "sparc-*-*", I didn't
test the trivial change in that test-case.