Tests: print contents of *eglot ...* buffers in batch mode.

Tests: print contents of *eglot ...* buffers in batch mode.

Useful for the CI on github. To be able to see more of the context of
a failure.

* eglot.el (eglot-server-initialized-hook): Changed semantics. Now
called when an instance of `eglot-lsp-server' is created as part of
the "connect to server" flow. Previously, there was no difference
between this hook and `eglot-connect-hook' which continues to be run
once a connection was successfully established. The
`eglot-server-initialized-hook' will now capture ALL server instances
including those that failed to be started. This change was necessary
to make the test suite be able to dump the output of processes that
fail to start when running the test suite in batch mode ("make check"
and the CI.) In PR https://github.com/joaotavora/eglot/issues/448 it was decided that it is ok to change the
semantics of this hook rather than introducing a new hook.
(eglot--connect): Change place of where the hook is run.
(eglot-connect-hook): Initialized now with
`eglot-signal-didChangeConfiguration' which was kept in
`eglot-server-initialized-hook' before.

* eglot-tests.el (eglot--call-with-fixture): Use
`eglot-server-initialized-hook' rather than `eglot-connect-hook'. And
dump the contents of the *EGLOT ...* buffers when run in
`noninteractive' (i.e. batch) mode.
(eglot--cleanup-after-test): New auxiliary function. Extracted
verbatim out of `eglot--call-with-fixture` in order to lower the
latter's LOC.