Add --coverage option to build scripts for tests (microsoft#54499)

.c8rc.json

@@ -0,0 +1,7 @@
+{
+    "reporter": ["lcovonly", "cobertura"],
+    "src": "src",
+    "include": ["src/**", "built/local/**"],
+    "exclude": ["**/node_modules/**"],
+    "mergeAsync": true
+}

Herebyfile.mjs

@@ -577,6 +577,7 @@ export const runTests = task({
 // task("runtests").flags = {
 //     "-t --tests=<regex>": "Pattern for tests to run.",
 //     "   --failed": "Runs tests listed in '.failed-tests'.",
+//     "   --coverage": "Generate test coverage using c8",
 //     "-r --reporter=<reporter>": "The mocha reporter to use.",
 //     "-i --break": "Runs tests in inspector mode (NodeJS 8 and later)",
 //     "   --keepFailed": "Keep tests in .failed-tests even if they pass",
@@ -714,6 +715,7 @@ export const runTestsParallel = task({
 });
 
 // task("runtests-parallel").flags = {
+//     "   --coverage": "Generate test coverage using c8",
 //     "   --light": "Run tests in light mode (fewer verifications, but tests run faster).",
 //     "   --keepFailed": "Keep tests in .failed-tests even if they pass.",
 //     "   --dirty": "Run tests without first cleaning test output directories.",