ci: Run devtools tests whenever we run unit tests (#38614)

this patch updates linux.yml, mac.yml, and windows.yml to run the
devtools test suite (#36325), whenever unit tests are enabled in those
workflows. plus three changes that speed up the 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)

finally 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 effectively adds 44 tests to CI
Fixes: #36325

---------

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

.github/workflows/linux.yml

@@ -190,6 +190,9 @@ jobs:
           timeout_minutes: 20
           max_attempts: 2 # https://github.com/servo/servo/issues/30683
           command: ./mach test-unit --${{ inputs.profile }}
+      - name: Devtools tests
+        if: ${{ inputs.unit-tests }}
+        run: ./mach test-devtools --${{ inputs.profile }}
       - name: Build libservo with examples
         if: ${{ inputs.build-libservo }}
         continue-on-error: true

.github/workflows/mac.yml

@@ -174,6 +174,9 @@ jobs:
           timeout_minutes: 40 # https://github.com/servo/servo/issues/30275
           max_attempts: 3 # https://github.com/servo/servo/issues/30683
           command: ./mach test-unit --${{ inputs.profile }}
+      - name: Devtools tests
+        if: ${{ inputs.unit-tests }}
+        run: ./mach test-devtools --${{ inputs.profile }}
       - name: Build libservo with examples
         if: ${{ inputs.build-libservo }}
         continue-on-error: true

.github/workflows/windows.yml

@@ -187,6 +187,9 @@ jobs:
           timeout_minutes: 30
           max_attempts: 3 # https://github.com/servo/servo/issues/30683
           command: .\mach test-unit --${{ inputs.profile }} -- -- --test-threads=1
+      - name: Devtools tests
+        if: ${{ inputs.unit-tests }}
+        run: .\mach test-devtools --${{ inputs.profile }}
       - name: Build libservo with examples
         if: ${{ inputs.build-libservo }}
         continue-on-error: true