Unit tests: 5 failures related to ~ expansion

[mgorny]

		=== unit tests ===

Schedule of variations:
    unix

Running target unix
Using /usr/share/dejagnu/baseboards/unix.exp as board description file for target.
Using /usr/share/dejagnu/config/unix.exp as generic interface file for target.
Using ./config/default.exp as tool-and-target-specific interface file.
Running ./unit/__expand_tilde_by_ref.exp ...
FAIL: ~user should return /home/user
FAIL: ~user/bar should return /home/user/bar
FAIL: ~user/$HOME should return /home/user/$HOME
FAIL: '~user/a  b' should return '/home/user/a  b'
FAIL: ~user/* should return /home/user/*
Running ./unit/_count_args.exp ...
Running ./unit/_filedir.exp ...
Running ./unit/_get_comp_words_by_ref.exp ...
Running ./unit/_get_cword.exp ...
Running ./unit/_init_completion.exp ...
Running ./unit/_known_hosts_real.exp ...
Running ./unit/_parse_help.exp ...
Running ./unit/_parse_usage.exp ...
Running ./unit/_tilde.exp ...
Running ./unit/_variables.exp ...
Running ./unit/compgen.exp ...
Running ./unit/find_unique_completion_pair.exp ...
Running ./unit/quote.exp ...

		=== unit Summary ===

# of expected passes		201
# of unexpected failures	5
# of unsupported tests		1
/tmp/bash-completion/test, bash-4.4.12(1)-release

log/unit.log: https://gist.github.com/mgorny/f82dc507f21387eccd0e441ac81c9f86

The relevant bits seem to be e.g.:

/@echo "$HOME"
./config
/@echo $?
0
/@echo "$USER"
mgorny

[...]

/@var="~mgorny"; __expand_tilde_by_ref var; printf "%s\n" "$var"
/home/mgorny
/@FAIL: ~user should return /home/user
^C
/@var='~/foo'; __expand_tilde_by_ref var; printf "%s\n" "$var"
./config/foo
/@PASS: ~/foo should return /home/user/foo

Which indicates that $HOME gets overriden for some reason. FWICS, it happens inside config/bashrc, and is due to c96f432. I can confirm that reverting this commit (i.e. removing the HOME override) makes the tests succeed.