🧪 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.

.gitmodules

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+[submodule "test/bats"]
+	path = test/bats
+	url = https://github.com/bats-core/bats-core.git
+[submodule "test/test_helper/bats-support"]
+	path = test/test_helper/bats-support
+	url = https://github.com/bats-core/bats-support.git
+[submodule "test/test_helper/bats-assert"]
+	path = test/test_helper/bats-assert
+	url = https://github.com/bats-core/bats-assert.git

test/bats

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+Subproject commit 26c9b18983c3ce4cf24c6f6ee942abd8b1c3ee18

test/bemoji.bats

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+#!/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
+}

test/test_helper/bats-assert

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+Subproject commit ffe84ea5dd43b568851549b3e241db150c12929c

test/test_helper/bats-support

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+Subproject commit 3c8fadc5097c9acfc96d836dced2bb598e48b009