Add test.sh. (#173)

This emulates the test matrix that gets run in CI, making it easier to
find failures locally.

There was one conflict in the matrix: when JSON is enabled and TLS is
disabled, two of the doctests would fail. This was previous worked
around as an exclude in the github workflow. I changed the JSON doctest
to use HTTP instead.

test.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+set -eu
+
+export RUST_BACKTRACE=1
+export RUSTFLAGS="-D dead_code -D unused-variables -D unused"
+
+for tls in "" tls native-tls ; do
+  for feature in "" json charset cookies socks-proxy ; do
+    for what in --doc --tests ; do
+      if ! cargo test "${what}" --no-default-features --features "${tls} ${feature}" ; then
+        echo Command failed: cargo test \"${what}\" --no-default-features --features \"${tls} ${feature}\"
+        exit 1
+      fi
+    done
+  done
+done