[gdb/testsuite] Fix reverse attribute in tuiterm

[gdb/testsuite] Fix reverse attribute in tuiterm

I noticed in proc Term::_csi_m arguments that while parameters 7 and 27 are
supposed to set the reverse attribute to 1 and 0, in fact it's set to 1 in
both cases:
...
 		    7 {
			set _attrs(reverse) 1
		    }
  ...
		    27 {
			set _attrs(reverse) 1
 		    }
...

Fix this and add a regression test in gdb.tui/tuiterm.exp.

Tested on x86_64-linux.