test: fix "Extension Tests (current file)" task on Windows #1057

Handle backslashes.

Author: justinmk3

third-party/test/index.ts

@@ -122,7 +122,7 @@ function run(testsRoot, clb): any {
     // 2020-03-24: Amazon addition.
     // VSCode refuses to unset this value, so it gets set for each task and null is converted to a string
     const testFile = process.env["TEST_FILE"] === 'null' ? undefined : process.env["TEST_FILE"]
-    const testFilePath = testFile?.replace(/^src\/test\//, "")?.concat('.js')
+    const testFilePath = testFile?.replace(/^src[\\\/]test[\\\/]/, '')?.concat('.js')
 
     const globalSetupPath = paths.join(testsRoot, 'globalSetup.test.js')
     if (testFilePath && fs.existsSync(globalSetupPath)) {