Allow TechDocsAddonTester to be extended.

Signed-off-by: Eric Peterson <ericpeterson@spotify.com>

.changeset/techdocs-money-banana-stand.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-techdocs-addons-test-utils': patch
+---
+
+The `TechDocsAddonTester` class may now be extended if custom test configuration is needed.

plugins/techdocs-addons-test-utils/api-report.md

@@ -12,6 +12,7 @@ import { TechDocsMetadata } from '@backstage/plugin-techdocs-react';
 
 // @public (undocumented)
 export class TechDocsAddonTester {
+  protected constructor(addons: ReactElement[]);
   // (undocumented)
   atPath(path: string): this;
   // (undocumented)

plugins/techdocs-addons-test-utils/src/test-utils.tsx

@@ -119,7 +119,8 @@ export class TechDocsAddonTester {
     return new TechDocsAddonTester(addons);
   }
 
-  private constructor(addons: ReactElement[]) {
+  // Protected in order to allow extension but not direct instantiation.
+  protected constructor(addons: ReactElement[]) {
     this.addons = addons;
   }