Fix emNET support and add tests

The emNET `wolfSSL_LastError` branches were incorrect. The second one
was never hit and would never compile. The first one inverts error codes
that should not be inverted.

This fixes that code and adds a test with a shim layer to test emNET
calls without using emNET.

.github/workflows/emnet-nonblock.yml

@@ -0,0 +1,56 @@
+name: emNET non-blocking handshake test
+
+# START OF COMMON SECTION
+on:
+  push:
+    branches: [ 'master', 'main', 'release/**' ]
+  pull_request:
+    branches: [ '*' ]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+# END OF COMMON SECTION
+
+# Build wolfSSL with -DWOLFSSL_EMNET using the clean-room shim in
+# tests/emnet/ (IP/IP.h + emnet_shim.c), link a non-blocking TLS 1.3
+# handshake test with -Wl,--wrap=recv,--wrap=send, and run it. The
+# test exercises the WOLFSSL_EMNET path in wolfSSL_LastError() and
+# asserts that would-block events surface as WANT_READ/WANT_WRITE and
+# the handshake completes, guarding against regressions in the emNET
+# error-translation logic.
+
+jobs:
+  emnet_nonblock:
+    name: wolfSSL emNET non-blocking handshake
+    if: github.repository_owner == 'wolfssl'
+    runs-on: ubuntu-24.04
+    timeout-minutes: 20
+    steps:
+      - name: Checkout wolfSSL
+        uses: actions/checkout@v4
+
+      - name: Install build deps
+        uses: ./.github/actions/install-apt-deps
+        with:
+          packages: autoconf automake libtool build-essential
+
+      - name: Bootstrap
+        run: ./autogen.sh
+
+      - name: Configure wolfSSL (WOLFSSL_EMNET + emNET shim headers)
+        run: |
+          ./configure \
+            --enable-static --disable-shared \
+            --enable-tls13 --disable-oldtls \
+            --enable-ecc --disable-examples \
+            CFLAGS="-DWOLFSSL_EMNET -I$(pwd)/tests/emnet"
+
+      - name: Build wolfSSL
+        run: make -j$(nproc)
+
+      - name: Build emNET non-blocking test
+        run: make -C tests/emnet
+
+      - name: Run emNET non-blocking test
+        run: make -C tests/emnet run