test/sbom/spdx: test using custom license index file

Add unit test for testing the use of custom license index file with
`SpdxLicenseExpressionFactory`.

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

test/data/spdx/README.md

@@ -1,3 +1,4 @@
 # This directory contains SPDX-related files used for testing
 
 - `spdx-schema-v2.3.1.json` is the SPDX 2.3.1 JSON schema file from the [spdx-spec upstream](https://github.com/spdx/spdx-spec/blob/62f3e7cbd448a19f0099fdbd707d8b9b4fbddfdd/schemas/spdx-schema.json)
+- `custom-license-index.json` is a custom license DB JSON file which includes only definition for `MIT` license and a new non-existent license `GPLv2` at the end of the file. The latest actual license DB JSON files can be downloaded from the [ScanCode LicenseDB JSON index](https://scancode-licensedb.aboutcode.org/index.json). See also [SPDX licenses](https://spdx.org/licenses/) and [ScanCode LicenseDB help](https://scancode-licensedb.aboutcode.org/help.html).

test/data/spdx/custom-license-index.json

@@ -0,0 +1,31 @@
+[
+  {
+    "license_key": "mit",
+    "category": "Permissive",
+    "spdx_license_key": "MIT",
+    "other_spdx_license_keys": [
+      "LicenseRef-MIT-Bootstrap",
+      "LicenseRef-MIT-Discord",
+      "LicenseRef-MIT-TC",
+      "LicenseRef-MIT-Diehl"
+    ],
+    "is_exception": false,
+    "is_deprecated": false,
+    "json": "mit.json",
+    "yaml": "mit.yml",
+    "html": "mit.html",
+    "license": "mit.LICENSE"
+  },
+  {
+    "license_key": "GPLv2",
+    "category": "Permissive",
+    "spdx_license_key": "GPLv2",
+    "other_spdx_license_keys": [],
+    "is_exception": false,
+    "is_deprecated": false,
+    "json": "GPLv2.json",
+    "yaml": "GPLv2.yml",
+    "html": "GPLv2.html",
+    "license": "GPLv2.LICENSE"
+  }
+]