Add draft source map spec tests

This commit adds a submodule for the draft source map spec test repo and adds a
new test file that runs each of the test cases. Some test cases that have known
failures are skipped, with a comment explaining why.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "test/source-map-tests"]
+	path = test/source-map-tests
+	url = https://github.com/takikawa/source-map-tests.git

test/test-spec-tests.js

@@ -0,0 +1,91 @@
+/* -*- Mode: js; js-indent-level: 2; -*- */
+/*
+ * Copyright 2024 Mozilla Foundation and contributors
+ * Licensed under the New BSD license. See LICENSE or:
+ * http://opensource.org/licenses/BSD-3-Clause
+ */
+
+const fs = require('node:fs/promises');
+const SourceMapConsumer =
+  require("../lib/source-map-consumer").SourceMapConsumer;
+
+const sourceMapSpecTests = require("./source-map-tests/source-map-spec-tests.json");
+
+async function readJSON(path) {
+  const file = await fs.open(require.resolve(path));
+  const json = JSON.parse(await file.readFile());
+  file.close();
+  return json;
+}
+
+// Known failures due to intentional implementation choices or due to bugs.
+const skippedTests = [
+  // Versions are explicitly checked a bit loosely.
+  "versionNumericString",
+  // Stricter sources array checking isn't implemented.
+  "sourcesNotStringOrNull",
+  "sourcesAndSourcesContentBothNull",
+  // Stricter names array checking isn't implemented.
+  "namesMissing",
+  "namesNotString",
+  // This check isn't as strict in this library.
+  "invalidMappingNotAString1",
+  // A mapping segment with no fields is technically invalid in the spec.
+  "invalidMappingSegmentWithZeroFields",
+  // These tests fail due to imprecision in the spec about the 32-bit limit.
+  "invalidMappingSegmentWithColumnExceeding32Bits",
+  "invalidMappingSegmentWithOriginalLineExceeding32Bits",
+  "invalidMappingSegmentWithOriginalColumnExceeding32Bits",
+  // A large VLQ that should parse, but currently does not.
+  "validMappingLargeVLQ",
+  // The library currently doesn't check the types of offset lines/columns.
+  "indexMapOffsetLineWrongType",
+  "indexMapOffsetColumnWrongType",
+  // The spec is not totally clear about this case.
+  "indexMapInvalidBaseMappings",
+  // The spec's definition of overlap can be refined
+  "indexMapInvalidOverlap",
+  // Not clear if this test makes sense, but spec isn't clear on behavior
+  "validMappingNullSources"
+]
+
+async function testMappingAction(assert, rawSourceMap, action) {
+  return SourceMapConsumer.with(rawSourceMap, null, (consumer) => {
+    let mappedPosition = consumer.generatedPositionFor({
+      source: action.originalSource,
+      line: action.originalLine + 1,
+      column: action.originalColumn
+    });
+
+    assert.equal(mappedPosition.line, action.generatedLine + 1, `generated line didn't match, expected ${action.generatedLine + 1} got ${mappedPosition.line}`);
+    assert.equal(mappedPosition.column, action.generatedColumn, `generated column didn't match, expected ${action.generatedColumn} got ${mappedPosition.column}`);
+  });
+}
+
+for (let testCase of sourceMapSpecTests.tests) {
+  if (skippedTests.includes(testCase.name))
+    continue;
+  exports[`test from source map spec tests, name: ${testCase.name}`] =
+    async function (assert) {
+      const json = await readJSON(`./source-map-tests/resources/${testCase.sourceMapFile}`);
+      let sourceMapFailed = false;
+      try {
+        const map = await new SourceMapConsumer(json);
+        map.eachMapping(() => {});
+        map.destroy();
+      } catch (exn) {
+        if (testCase.sourceMapIsValid)
+          assert.fail("Expected valid source map but failed to load successfully: " + exn.message);
+        return;
+      }
+      if (!testCase.sourceMapIsValid)
+        assert.fail("Expected invalid source map but loaded successfully");
+      if (testCase.testActions) {
+        for (let testAction of testCase.testActions) {
+          if (testAction.actionType == "checkMapping") {
+            await testMappingAction(assert, json, testAction);
+          }
+        }
+      }
+    };
+};