patch 9.0.0956: terminal tests fail when using key with modifier

Problem:    Terminal tests fail when using key with modifier.
Solution:   Use the modifyOtherKeys encoding when using RunVimInTerminal().

src/testdir/test_mapping.vim

@@ -584,7 +584,7 @@ func Test_expr_map_restore_cursor()
   END
   call writefile(lines, 'XtestExprMap', 'D')
   let buf = RunVimInTerminal('-S XtestExprMap', #{rows: 10})
-  call term_sendkeys(buf, "\<C-B>")
+  call term_sendkeys(buf, GetEscCodeWithModifier('C', 'B'))
   call VerifyScreenDump(buf, 'Test_map_expr_1', {})
 
   " clean up