test_pause giving '__git_ps1: not found' warning

I was trying to refine a test_expect_failure test [1] and tried inserting a `test_pause &&` test line [2].

I then found, when it paused, I was repeatedly given the warning line
     /bin/sh: 1: __git_ps1: not found
in the terminal until I expected the test shell.

my PS1 is working normally in the terminal, but not here.  Is this expected, or do I need to set up anything else?

Normally I'm on Git for Windows, but this was on my old laptop (Acer 7741 i5 4GB ram..) converted to Ubuntu 20.04, which I use when away.

The basic sequence was

~$ cd repos/git ~/repos/git (doctrunc *)$ cd t ~/repos/git/t (doctrunc *)$ ./t4205-log-pretty-formats.sh  -i -x -v Initialized empty Git repository in /home/philip/repos/git/t/trash directory.t4205-log-pretty-formats/.git/

[... ...]

+ git log --format=%<(5,mtrunc)%s -4
+ test_pause
+ PAUSE_TERM=dumb
+ PAUSE_SHELL=/bin/sh
+ PAUSE_HOME=/home/philip/repos/git/t/trash 
directory.t4205-log-pretty-formats
+ test 0 != 0
+ TERM=dumb HOME=/home/philip/repos/git/t/trash 
directory.t4205-log-pretty-formats /bin/sh
/bin/sh: 1: __git_ps1: not found
\w$ git status
On branch source-b
[...]
nothing added to commit but untracked files present (use "git add" to track)
/bin/sh: 1: __git_ps1: not found

I added the gits status to see when/where the warning was emitted.

[1] https://github.com/PhilipOakley/git/blob/doctrunc/t/t4205-log-pretty-formats.sh#L1021-L1046 [2] https://github.com/git/git/blob/master/t/test-lib-functions.sh#L137-L188

Any suggestions as to how to set this up correctly to avoid the warning? -- Philip