[gdb/testsuite] Fix gdb.base/empty-host-env-vars.exp

On aarch64-linux (debian testing) with test-case
gdb.base/empty-host-env-vars.exp I ran into:
...
(gdb) show index-cache directory^M
The directory of the index cache is "/home/linux/.cache/gdb".^M
(gdb) FAIL: $exp: env_var_name=HOME: show index-cache directory
...

Without changing any environment variables, the value of the index-cache dir
is:
...
$ gdb -q -batch -ex "show index-cache directory"
The directory of the index cache is "/home/linux/.cache/gdb".
...
and the expectation of the test-case is that setting HOME to empty will
produce an empty dir, but what it actually produces is:
...
$ HOME= gdb -q -batch -ex "show index-cache directory"
The directory of the index cache is "/home/linux/.cache/gdb".
...

There's nothing wrong with that behaviour, the dir is simply constructed using
XDG_CACHE_HOME which happens to be explictly set to its default value
$HOME/.cache [1]:
...
$ echo $XDG_CACHE_HOME
/home/linux/.cache
...
and indeed also setting that variable to empty gets us the expected empty dir:
...
$ XDG_CACHE_HOME= HOME= gdb -q -batch -ex "show index-cache directory"
gdb: warning: Couldn't determine a path for the index cache directory.
The directory of the index cache is "".
...

Furthermore, the test-case assumption that setting variables to empty either
produces the original dir or an empty dir is incorrect.

Say that XDG_CACHE_HOME has a non-default value:
...
$ echo $XDG_CACHE_HOME
/home/linux/my-xdg-cache-home
$ gdb -q -batch -ex "show index-cache directory"
The directory of the index cache is "/home/linux/my-xdg-cache-home/gdb".
...
then setting that variable to empty:
...
$ XDG_CACHE_HOME= gdb -q -batch -ex "show index-cache directory"
The directory of the index cache is "/home/linux/.cache/gdb".
...
does change the value of the dir.

Fix this by making the test-case less specific.

While we're at it, factor out regexps re_pre and re_post to make regexps more
readable, and use string_to_regexp to reduce quoting.

Tested on aarch64-linux.

PR testsuite/32132
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32132

[1] https://specifications.freedesktop.org/basedir-spec/latest/index.html#variables

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