ci: fix FUSE mounts against the new runner image (#10484)

* ci: restore the setuid bit on a shadowed fusermount3

Newer ubuntu-22.04 runner images carry a source-built fusermount3 in
/usr/local/bin that shadows the distro one in PATH and is not setuid
root. go-fuse looks the helper up through PATH, so every unprivileged
mount fails with "mount failed: Operation not permitted".

* test: fail a fuse test as soon as its mount process dies

A mount that cannot mount at all exits within a second, but the harness
still waited out the 30s readiness timeout and then reported "mount
point not ready within timeout", leaving the real cause buried in the
log tail. Watch the child processes and report their exit instead.

* mount: report a failed mount without a goroutine dump

A mount failure is an environment problem - no /dev/fuse, fusermount not
setuid, stale mount point - and the all-goroutine stack dump Fatalf adds
buries the one line that says so.

.github/workflows/fuse-integration.yml

@@ -45,6 +45,18 @@ jobs:
         # Allow non-root FUSE mounts with allow_other
         echo 'user_allow_other' | sudo tee -a /etc/fuse.conf
         sudo chmod 644 /etc/fuse.conf
+        # Some runner images carry a second, source-built fusermount3 in
+        # /usr/local/bin that shadows the distro one in PATH without its setuid
+        # bit, and every unprivileged mount then fails with EPERM. go-fuse takes
+        # the first fusermount3 in PATH, so repair that one.
+        fusermount_bin=$(command -v fusermount3 || true)
+        if [ -n "$fusermount_bin" ]; then
+          if [ ! -u "$fusermount_bin" ]; then
+            sudo chown root:root "$fusermount_bin"
+            sudo chmod u+s "$fusermount_bin"
+          fi
+          ls -l "$fusermount_bin"
+        fi
         # Verify FUSE installation
         fusermount3 --version || fusermount --version || true
         ls -la /dev/fuse