Co-authored-by: zeertzjq <zeertzjq@outlook.com>

`!did_throw` doesn't exactly imply `!current_exception`, as `did_throw = false`
is sometimes used to defer exception handling for later (without forgetting the
exception). E.g: uncaught exception handling in `do_cmdline()` may be deferred
to a different call (e.g: when `try_level > 0`).

In #7881, `current_exception = NULL` in `do_cmdline()` is used as an analogue of
`did_throw = false`, but also causes the pending exception to be lost, which
also leaks as `discard_exception()` wasn't used.

It may be possible to fix this by saving/restoring `current_exception`, but
handling all of `did_throw`'s edge cases seems messier. Maybe not worth
diverging over.

This fix also uncovers a `man_spec.lua` bug on Windows: exceptions are thrown
due to Windows missing `man`, but they're lost; skip these tests if `man` isn't
executable.

Problem:
q in "$MANPAGER mode" does not quit Nvim. This is because
ftplugin/man.vim creates its own mapping:
    nnoremap <silent> <buffer> <nowait> q :lclose<CR><C-W>c
which overrides the one set by the autoload file when using :Man!
("$MANPAGER mode")

Solution:
Set b:pager during "$MANPAGER mode" so that ftplugin/man.vim can set the
mapping correctly.

Fixes #18281
Ref #17791

Helped-by: Gregory Anders <8965202+gpanders@users.noreply.github.com>

It is perfectly fine and expected to detach from the screen just by
the UI disconnecting from nvim or exiting nvim. Just keep detach() in
screen_basic_spec, to get some coverage of the detach method itself.

This avoids hang on failure in many situations (though one could argue
that detach() should be "fast", or at least "as fast as resize",
which works in press-return already).

Never use detach() just to change the size of the screen, try_resize()
method exists for that specifically.

"attr_ignore" is an anti-pattern, with snapshot_util()
just include all the highlights already.

The call to plugin_helpers.reset() is redundant with the clear() call
above it.  Probably just a copy-paste mistake.

Avoids exit_event race #8813.

Helped-by: Björn Linse <bjorn.linse@gmail.com>

closes #10267

Lua (not LuaJIT) complains about the "^[[" strings inside the expect,
since it sees them as nested quotes.  Change the quoting to [=[ ]=] to
avoid the issue.