🧪 Add simple test framework

Now that the project is finding a little adoption it is really high time
to bring some tests into the code to ensure everything works as it
should.
This first version will only provide a simple test harness from which
tests can slowly be written for other parts of the program.

test/bemoji.bats

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+
+setup() {
+    load 'test_helper/bats-support/load'
+    load 'test_helper/bats-assert/load'
+
+    # make bemoji executable from anywhere relative to current testfile
+    TEST_DIR="$( cd "$( dirname "$BATS_TEST_FILENAME" )" >/dev/null 2>&1 && pwd )"
+    PATH="$TEST_DIR/..:$PATH"
+}
+
+@test "test can run script" {
+    # closing FD3 manually to prevent hangs, see
+    # https://bats-core.readthedocs.io/en/stable/writing-tests.html#file-descriptor-3-read-this-if-bats-hangs
+    run bemoji 3>&-
+    assert_success
+}