devtools: Reland test improvements from #38614 minus CI changes (#38933)

#38614 was reverted due to CI flakiness, but it also included several
improvements to the devtools tests. this patch relands those
improvements, described below.

we make three changes that speed up the devtools tests from 73 → 65 → 56
→ 51 seconds:

- we replace the hardcoded sleep(1) after starting servoshell with a
loop that waits until the devtools port is open (this also fixes
intermittent failures when servoshell starts too slowly, especially on
macOS)
- we start the internal web servers once, and reuse them across all
tests
- we run servoshell in headless mode (this is also required because most
CI runners have no GUI)

and we fix two bugs that cause very noisy but not very interesting error
messages:

- in the test code, we use a [context
manager](https://docs.python.org/3/reference/datamodel.html#context-managers)
to ensure the devtools client is disconnected unconditionally, even if
test methods or assert helper methods raise exceptions (this was causing
errors on all platforms)
- in the devtools server, we treat “connection reset” errors when
reading from the client like a normal EOF, rather than as a failure
(this was causing errors on Windows)

on self-hosted linux builds, there are still spurious error messages
like the following, but we can fix them later:

```
error: XDG_RUNTIME_DIR not set in the environment.
libEGL warning: egl: failed to create dri2 screen
```

Testing: this patch improves the devtools tests, but does not change
their coverage
Fixes: part of #36325

---------

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Signed-off-by: atbrakhi <atbrakhi@igalia.com>
Co-authored-by: atbrakhi <atbrakhi@igalia.com>

components/devtools/protocol.rs

@@ -5,7 +5,7 @@
 //! Low-level wire protocol implementation. Currently only supports
 //! [JSON packets](https://firefox-source-docs.mozilla.org/devtools/backend/protocol.html#json-packets).
 
-use std::io::{Read, Write};
+use std::io::{ErrorKind, Read, Write};
 use std::net::TcpStream;
 
 use log::debug;
@@ -49,7 +49,8 @@ impl JsonPacketStream for TcpStream {
         loop {
             let mut buf = [0];
             let byte = match self.read(&mut buf) {
-                Ok(0) => return Ok(None), // EOF
+                Ok(0) => return Ok(None),                                            // EOF
+                Err(e) if e.kind() == ErrorKind::ConnectionReset => return Ok(None), // EOF
                 Ok(1) => buf[0],
                 Ok(_) => unreachable!(),
                 Err(e) => return Err(e.to_string()),