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

When running the testsuite I have:

    Running .../gdb/testsuite/gdb.base/interp.exp ...
    DUPLICATE: gdb.base/interp.exp: interpreter-exec mi "-var-update *"

This is due to the fact that multiple successive instances of
gdb_test_multiple use 'pass $cmd', but one of them forgets to reset $cmd
to a new test name.

Fix by using 'pass $gdb_test_name', given that the gdb_test_name is set
by gdb_test_multiple.

While fixing this, this patch refactors all occurrences of the following
pattern:

    set cmd foo
    gdb_test_multiple $cmd $cmd {
        -re ... {
            pass $cmd
        }
    }

into

    gdb_test_multiple foo "" {
        -re ... {
            pass $gdb_test_name
         }
    }

This makes this test file coherent in its use of $gdb_test_name.

Tested on x86_64-linux.

gdb/testsuite/gdb.base/interp.exp

@@ -22,10 +22,9 @@ if { [prepare_for_testing "failed to prepare" ${testfile} $srcfile {debug}] } {
 }
 
 # Do not use gdb_test for this test, since it has two prompts.
-set cmd "interpreter-exec mi \"-var-update *\""
-gdb_test_multiple $cmd $cmd {
+gdb_test_multiple "interpreter-exec mi \"-var-update *\"" "" {
     -re "\\^done,changelist=\\\[\\\]\r\n$gdb_prompt " {
-	pass "$cmd"
+	pass $gdb_test_name
 	gdb_expect 1 {
 	    -re "\r\n$gdb_prompt $" { }
 	}
@@ -37,47 +36,43 @@ gdb_test "interpreter-exec console \"show version\"" "GNU gdb .*"
 gdb_test_multiple "interpreter-exec mi \"-break-insert --thread a\"" \
     "regression test for mi_parse crash" {
 	-re ".error,msg=.Invalid value for the '--thread' option.\r\n$gdb_prompt " {
-	    pass "$cmd"
+	    pass $gdb_test_name
 	    gdb_expect 1 {
 		-re "\r\n$gdb_prompt $" { }
 	    }
 	}
     }
 
-set cmd "interpreter-exec mi \"-stack-info-frame\""
-gdb_test_multiple $cmd $cmd {
+gdb_test_multiple "interpreter-exec mi \"-stack-info-frame\"" "" {
     -re ".error,msg=.No registers\..\r\n$gdb_prompt " {
-	pass "$cmd"
+	pass $gdb_test_name
 	gdb_expect 1 {
 	    -re "\r\n$gdb_prompt $" { }
 	}
     }
 }
 
-set cmd "interpreter-exec mi1 \"-break-insert main\""
-gdb_test_multiple $cmd $cmd {
+gdb_test_multiple "interpreter-exec mi1 \"-break-insert main\"" "" {
     -re ".done.bkpt=.number=.\[0-9\]\[^\n\]+\r\n$gdb_prompt " {
-	pass "$cmd"
+	pass $gdb_test_name
 	gdb_expect 1 {
 	    -re "\r\n$gdb_prompt $" { }
 	}
     }
 }
 
-set cmd "interpreter-exec mi2 \"-break-insert main\""
-gdb_test_multiple $cmd $cmd {
+gdb_test_multiple "interpreter-exec mi2 \"-break-insert main\"" "" {
     -re ".done.bkpt=.number=.\[0-9\]\[^\n\]+\r\n$gdb_prompt " {
-	pass "$cmd"
+	pass $gdb_test_name
 	gdb_expect 1 {
 	    -re "\r\n$gdb_prompt $" { }
 	}
     }
 }
 
-set cmd "interpreter-exec mi3 \"-break-insert main\""
-gdb_test_multiple $cmd $cmd {
+gdb_test_multiple "interpreter-exec mi3 \"-break-insert main\"" "" {
     -re ".done.bkpt=.number=.\[0-9\]\[^\n\]+\r\n$gdb_prompt " {
-	pass "$cmd"
+	pass $gdb_test_name
 	gdb_expect 1 {
 	    -re "\r\n$gdb_prompt $" { }
 	}