Fix fail in gdb.server/solib-list.exp

Fix fail in gdb.server/solib-list.exp

If I run single test solib-list.exp, it is OK.  If I run two, as below,
there are fails,

$ make check RUNTESTFLAGS="server-run.exp solib-list.exp"
FAIL: gdb.server/solib-list.exp: non-stop 0: continue (the program exited)
FAIL: gdb.server/solib-list.exp: non-stop 0: p libvar
FAIL: gdb.server/solib-list.exp: non-stop 1: continue (the program exited)
FAIL: gdb.server/solib-list.exp: non-stop 1: p libvar

in gdb.log,

/scratch/yao/gdb/build-git/x86_64/gdb/testsuite/../../gdb/gdbserver/gdbserver --once :2347 /scratch/yao/gdb/build-git/x86_64/gdb/testsuite/outputs/gdb.server/server-run/server-run /lib64/ld-linux-x86-64.so.2 /scratch/yao/gdb/build-git/x86_64/gdb/testsuite/outputs/gdb.server/solib-list/solib-list

server-run is spawned, which is wrong.  If I only run solib-list.exp, ld-linux
is spawned, which is right.

/scratch/yao/gdb/build-git/x86_64/gdb/testsuite/../../gdb/gdbserver/gdbserver --once :2346 /lib64/ld-linux-x86-64.so.2 /scratch/yao/gdb/build-git/x86_64/gdb/testsuite/outputs/gdb.server/solib-list/solib-list

in test, we spawn gdbserver this way,

    # Note we pass ${interp_system}, the program gdbserver spawns, as
    # argument here, instead of using gdb_load, because we don't want
    # to download the interpreter to the target (it's already there)
    # or to the test output directory.
    set res [gdbserver_spawn "${interp_system} ${remote_binfile}"]

in gdbserver_spawn -> gdbserver_download_current_prog, if
last_loaded_file is set (when you run multiple tests), it is
returned.

This patch is to unset last_loaded_file in solib-list.exp.

gdb/testsuite:

2016-07-21  Yao Qi  <yao.qi@linaro.org>

	* gdb.server/solib-list.exp: Unset last_loaded_file.