gdb/testsuite: Remove duplicates from gdb.base/unload.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/unload.exp ...
    DUPLICATE: gdb.base/unload.exp: continuing to unloaded libfile

Fix by adjusting the test name.

Tested on x86_64-linux.
diff --git a/gdb/testsuite/gdb.base/unload.exp b/gdb/testsuite/gdb.base/unload.exp
index a041dde..6033203 100644
--- a/gdb/testsuite/gdb.base/unload.exp
+++ b/gdb/testsuite/gdb.base/unload.exp
@@ -119,7 +119,7 @@
 
 gdb_test "continue" \
 "Continuing.*warning: Temporarily disabling breakpoints for.*${lib_syms}.*y-set-1.*" \
-"continuing to unloaded libfile"
+"continuing to unloaded libfile, two"
 
 set unloadshr_line2 [gdb_get_line_number "unloadshr2 break" ${libsrcfile2}]