Add regression testing for the custom assets functionality (#1829)

The test is following the same steps regular users are instructed to follow:
https://github.com/sharkdp/bat/blob/master/README.md#adding-new-syntaxes--language-definitions

tests/syntax-tests/test_custom_assets.sh

@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+set -o errexit -o nounset -o pipefail
+
+### ENVIRONMENT
+
+BAT_CONFIG_DIR=$(mktemp -d)
+export BAT_CONFIG_DIR
+
+BAT_CACHE_PATH=$(mktemp -d)
+export BAT_CACHE_PATH
+
+echo "
+BAT_CONFIG_DIR = ${BAT_CONFIG_DIR}
+BAT_CACHE_PATH = ${BAT_CACHE_PATH}
+"
+
+### HELPER VARS
+
+custom_syntax_args=(
+    "--language=BatTestCustomAssets"
+    "tests/syntax-tests/source/BatTestCustomAssets/NoColorsUnlessCustomAssetsAreUsed.battestcustomassets"
+)
+
+integrated_syntax_args=(
+    "--language=Rust"
+    "examples/simple.rs"
+)
+
+### HELPER FUNCTIONS
+
+echo_step() {
+    echo -e "\n== $1 =="
+}
+
+fail_test() {
+    echo -e "FAIL: $1"
+    exit 1
+}
+
+### TEST STEPS
+
+echo_step "TEST: Make sure 'BatTestCustomAssets' is not part of integrated syntaxes"
+bat -f "${custom_syntax_args[@]}" &&
+    fail_test "EXPECTED: 'unknown syntax' error ACTUAL: no error occured"
+
+echo_step "PREPARE: Install custom syntax 'BatTestCustomAssets'"
+custom_syntaxes_dir="$(bat --config-dir)/syntaxes"
+mkdir -p "${custom_syntaxes_dir}"
+cp -v "tests/syntax-tests/BatTestCustomAssets.sublime-syntax" \
+    "${custom_syntaxes_dir}/BatTestCustomAssets.sublime-syntax"
+
+echo_step "PREPARE: Build custom assets to enable 'BatTestCustomAssets' syntax"
+bat cache --build
+
+echo_step "TEST: 'BatTestCustomAssets' is a known syntax"
+bat -f "${custom_syntax_args[@]}" ||
+    fail_test "EXPECTED: syntax highlighting to work ACTUAL: there was an error"
+
+echo_step "TEST: The 'Rust' syntax is still available"
+bat -f "${integrated_syntax_args[@]}" ||
+    fail_test "EXPECTED: syntax highlighting still works with integrated assets ACTUAL: there was an error"
+
+echo_step "TEST: 'BatTestCustomAssets' is an unknown syntax with --no-custom-assets"
+bat -f --no-custom-assets "${custom_syntax_args[@]}" &&
+    fail_test "EXPECTED: 'unknown syntax' error because of --no-custom-assets ACTUAL: no error occured"
+
+echo_step "TEST: 'bat cache --clear' removes all files"
+bat cache --clear
+remaining_files=$(ls -A "${BAT_CACHE_PATH}")
+[ -z "${remaining_files}" ] ||
+    fail_test "EXPECTED: no files remain ACTUAL: some files remain:\n${remaining_files}"
+
+echo_step "CLEAN"
+rm -rv "${BAT_CONFIG_DIR}" "${BAT_CACHE_PATH}"