test: Provide stronger missing provider tests

The original code always ended up with no providers at all, which
triggered the expected error. But the point of the test was to test with
only a single missing required provider in each case.

extensions/historical-imagery/tests/historical-imagery_collection.test.mjs

@@ -97,9 +97,10 @@ o.spec('Historical Imagery Extension Collection', () => {
 
   o('Collection without required provider roles should fail validation', async () => {
     // given
-    for (const role of ['producer', 'licensor', 'processor', 'host']) {
+    const requiredProviderRoles = ['producer', 'licensor', 'processor', 'host'];
+    for (const role of requiredProviderRoles) {
       const example = JSON.parse(await fs.readFile(examplePath));
-      example.providers = example.providers.filter((provider) => (!role) in provider.roles);
+      example.providers = requiredProviderRoles.filter((entry) => entry !== role);
 
       // when
       let valid = validate(example);