patch 8.2.1954: Vim9: not all command modifiers are tested

Problem:    Vim9: not all command modifiers are tested.
Solution:   Add tests for "keep" modifiers.  Fix that marks are lost even
            though ":lockmarks" is used.
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index de4b806..185033f 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -1254,6 +1254,16 @@
 		if (read_linecount >= linecount)
 		    // move all marks from old lines to new lines
 		    mark_adjust(line1, line2, linecount, 0L);
+		else if (save_cmod_flags & CMOD_LOCKMARKS)
+		{
+		    // Move marks from the lines below the new lines down by
+		    // the number of lines lost.
+		    // Move marks from the lines that will be deleted to the
+		    // new lines and below.
+		    mark_adjust(line2 + 1, (linenr_T)MAXLNUM,
+					       linecount - read_linecount, 0L);
+		    mark_adjust(line1, line2, linecount, 0L);
+		}
 		else
 		{
 		    // move marks from old lines to new lines, delete marks