• are reporting a bug others will be able to reproduce and not asking a question. If you're not sure or want to ask a question do so on our Discourse: https://discourse.brew.sh
• ran a brew command and reproduced the problem with multiple formulae? If it's a problem with a single, official formula (not cask) please file this issue at Homebrew/homebrew-core: https://github.com/Homebrew/homebrew-core/issues/new/choose. If it's a brew cask problem please file this issue at https://github.com/Homebrew/homebrew-cask/issues/new/choose. If it's a tap (e.g. Homebrew/homebrew-php) problem please file this issue at the tap.
• ran brew update and can still reproduce the problem?
• ran brew doctor, fixed all issues and can still reproduce the problem?
• ran brew config and brew doctor and included their output with your issue?

Bug description

Several formulas see their test fail on macOS Catalina, due to impossibility to allocate a new TTY. One example is PuTTY:

$ brew test --verbose putty
[…]
==> ./command.sh
spawn /usr/local/Cellar/putty/0.72/bin/puttygen -t rsa -b 4096 -q -o test.key
The system has no more ptys.  Ask your system administrator to create more.
    while executing
"spawn /usr/local/Cellar/putty/0.72/bin/puttygen -t rsa -b 4096 -q -o test.key"
    (file "./command.sh" line 3)
Error: putty: failed

This failure reproduces in the shell within brew test. However, if I execute the same Expect script outside the sandbox, it succeeds:

$ cat command.sh
#!/usr/bin/expect -f
set timeout -1
spawn /usr/local/Cellar/putty/0.72/bin/puttygen -t rsa -b 4096 -q -o test.key
expect -exact "Enter passphrase to save key: "
send -- "Homebrew
"
expect -exact "
Re-enter passphrase to verify: "
send -- "Homebrew
"
expect eof
$ ./command.sh 
spawn /usr/local/Cellar/putty/0.72/bin/puttygen -t rsa -b 4096 -q -o test.key
Enter passphrase to save key: 
Re-enter passphrase to verify: