TAP test started using meson, can get a tcp port already used by another test.

Hello,

We running TAP tests which operates with a lot of instances. And some of
these tests randomly fail due "address already in use". It turned out
that the meson does not set environment variable MESON_BUILD_ROOT when
running the test() function [0]. As a result each test uses its own
"portlock" directory [1]. The small TAP test 001_portlock_env_test.pl
shows the test environment variables. As we can see MESON_BUILD_ROOT is
really undefined.

Can we explicitly set the MESON_BUILD_ROOT environment variable when
running a test? With included patch for the src/tools/testwrap file,
each instance gets an unique TCP port.

Thanks!

Best regards, Roman Zharkov

[0] https://mesonbuild.com/Reference-manual_functions.html#test
[1]
https://github.com/postgres/postgres/blob/7202d72787d3b93b692feae62ee963238580c877/src/test/perl/PostgreSQL/Test/Cluster.pm#L172