[gdb/testsuite] Move setting of Term::_last_char to Term::_insert

The variable Term::_last_char is meant to represent the last char inserted by
Term::_insert, but setting it is done alongside the only call to _insert in
lib/tuiterm.exp:
...
		_insert $expect_out(0,string)
		variable _last_char
		set _last_char [string index $expect_out(0,string) end]
...

Fix this by moving the setting of _last_char to inside _insert.

Tested on x86_64-linux.

gdb/testsuite/gdb.tui/tuiterm.exp

diff --git a/gdb/testsuite/gdb.tui/tuiterm.exp b/gdb/testsuite/gdb.tui/tuiterm.exp
index 6cd65f3..ed9478a 100644
--- a/gdb/testsuite/gdb.tui/tuiterm.exp
+++ b/gdb/testsuite/gdb.tui/tuiterm.exp
@@ -653,13 +653,21 @@
 }
 
 proc test_repeat { } {
-    Term::_move_cursor 2 1
-    set Term::_last_char X
+    Term::_move_cursor 0 1
 
-    Term::_csi_b 3
+    Term::_insert "xxX"
+    gdb_assert { $Term::_last_char == "X"  }
+    check "insert" {
+	"abcdefgh"
+	"xxXlmnop"
+	"qrstuvwx"
+	"yz01234 "
+    } 3 1
+
+    Term::_csi_b 2
     check "repeat" {
 	"abcdefgh"
-	"ijXXXnop"
+	"xxXXXnop"
 	"qrstuvwx"
 	"yz01234 "
     } 5 1

gdb/testsuite/lib/tuiterm.exp

diff --git a/gdb/testsuite/lib/tuiterm.exp b/gdb/testsuite/lib/tuiterm.exp
index 4ef3dc0..d622003 100644
--- a/gdb/testsuite/lib/tuiterm.exp
+++ b/gdb/testsuite/lib/tuiterm.exp
@@ -889,6 +889,9 @@
 		}
 	    }
 	}
+
+	variable _last_char
+	set _last_char [string index $str end]
     }
 }
 
@@ -1042,8 +1045,6 @@
 	}
 	-re "^\[^\x07\x08\x0a\x0d\x1b\]+" {
 	    _insert $expect_out(0,string)
-	    variable _last_char
-	    set _last_char [string index $expect_out(0,string) end]
 	}
 
 	timeout {