Upload JUnit summaries for our tests (#19330)

## Summary

Our `test.yml` jobs now upload JUnit reports as artifacts, using
nextest's JUnit export support. The plan is to begin consuming these
from astral-bot and producing useful difference comments on PRs.

## Test Plan

CI only, NFC.

---------

Signed-off-by: William Woodruff <william@yossarian.net>

.config/nextest.toml

@@ -12,6 +12,9 @@ final-status-level = "slow"
 failure-output = "immediate-final"
 fail-fast = false
 
+[profile.ci.junit]
+path = "junit.xml"
+
 [profile.ci-linux]
 inherits = "ci"
 

.github/workflows/test.yml

@@ -118,6 +118,15 @@ jobs:
           include-hidden-files: true
           retention-days: 14
 
+      - name: "Upload JUnit results"
+        if: always()
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: junit-results-linux
+          path: target/nextest/ci*/junit.xml
+          if-no-files-found: ignore
+          retention-days: 14
+
   cargo-test-macos:
     timeout-minutes: 20
     # Only run macOS tests on main without opt-in
@@ -185,6 +194,15 @@ jobs:
           include-hidden-files: true
           retention-days: 14
 
+      - name: "Upload JUnit results"
+        if: always()
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: junit-results-macos
+          path: target/nextest/ci*/junit.xml
+          if-no-files-found: ignore
+          retention-days: 14
+
   cargo-test-windows:
     timeout-minutes: 15
     runs-on: github-windows-2025-x86_64-16
@@ -264,3 +282,12 @@ jobs:
           if-no-files-found: ignore
           include-hidden-files: true
           retention-days: 14
+
+      - name: "Upload JUnit results"
+        if: always()
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
+        with:
+          name: junit-results-windows-${{ matrix.partition }}
+          path: target/nextest/ci*/junit.xml
+          if-no-files-found: ignore
+          retention-days: 14