fix `make check-stage1`

[pnkfelix]

PR #14831 broke it. We can fix it by making a slightly more flexible write! invocation in libtest/stats.rs

[pnkfelix]

but even with the fix suggested above, there are other problems. See e.g. my note on 2ed4734 (the lifetime shadowing variables fix that works by adding a leading quote to lifetimes) 2ed4734#commitcomment-6672036

[alexcrichton]

#14900 will close this issue

[alexcrichton]

Closing, #14900 landed.