[gdb/testsuite] Fix gdb.dwarf2/fission-with-type-unit.exp with remote host

When running test-case gdb.dwarf2/fission-with-type-unit.exp with a remote
host configuration, say host board local-remote-host and target board
remote-gdbserver-on-localhost, I run into:
...
(gdb) maint expand-symtabs^M
During symbol reading: Could not find DWO CU \
  fission-with-type-unit.dwo(0xf00d) referenced by CU at offset 0x2d7 \
  [in module /home/remote-host/fission-with-type-unit]^M
warning: Could not find DWO CU fission-with-type-unit.dwo(0xf00d) referenced \
  by CU at offset 0x2d7 [in module /home/remote-host/fission-with-type-unit]^M
(gdb) FAIL: gdb.dwarf2/fission-with-type-unit.exp: maint expand-symtabs
...

Fix this by adding the missing download to remote host of the .dwo file.

Tested by running make-check-all.sh on x86_64-linux.
diff --git a/gdb/testsuite/gdb.dwarf2/fission-with-type-unit.exp b/gdb/testsuite/gdb.dwarf2/fission-with-type-unit.exp
index a1b3bb7..0a02f7c 100644
--- a/gdb/testsuite/gdb.dwarf2/fission-with-type-unit.exp
+++ b/gdb/testsuite/gdb.dwarf2/fission-with-type-unit.exp
@@ -91,6 +91,10 @@
     return
 }
 
+if { [is_remote host] } {
+    gdb_remote_download host $dwo_file
+}
+
 clean_restart ${testfile}
 
 # This would cause an internal error.