patch 8.2.1555: not all tests are executed on Github Actions

Problem:    Not all tests are executed on Github Actions.
Solution:   Copy "src" to "src2" earlier. Recognize "src2" in a couple more
            places.  Add two tests to the list of flaky tests. (Ken Takata,
            closes #6798)

src/testdir/runtest.vim

@@ -382,7 +382,9 @@ endif
 
 " Names of flaky tests.
 let s:flaky_tests = [
+      \ 'Test_BufWrite_lockmarks()',
       \ 'Test_autocmd_SafeState()',
+      \ 'Test_bufunload_all()',
       \ 'Test_client_server()',
       \ 'Test_close_and_exit_cb()',
       \ 'Test_close_output_buffer()',