Bump apollo-server-testing from 2.7.0 to 2.7.2 in /backend

Sourced from apollo-server-testing's changelog.

v2.7.2

See complete versioning details.

• apollo-engine-reporting: Fix reporting errors from backend. (The support for federated metrics introduced in v2.7.0 did not properly handle GraphQL errors from the backend; all users of federated metrics should upgrade to this version.) [PR #3056](https://github-redirect.dependabot.com/apollographql/apollo-server/pull/3056) [Issue #3052](https://github-redirect.dependabot.com/apollographql/apollo-server/issues/3052)
• apollo-engine-reporting: Clean up SIGINT and SIGTERM handlers when EngineReportingAgent is stopped; fixes 'Possible EventEmitter memory leak detected' log. [PR #3090](https://github-redirect.dependabot.com/apollographql/apollo-server/pull/3090)

v2.7.1

See complete versioning details.

• apollo-engine-reporting: If an error is thrown by a custom variable transform function passed into the reporting option sendVariableValues: { transform: ... }, all variable values will be replaced with the string [PREDICATE_FUNCTION_ERROR].
• apollo-server-express: Typing fix for the connection property, which was missing from the ExpressContext interface. [PR #2959](https://github-redirect.dependabot.com/apollographql/apollo-server/pull/2959)
• @apollo/gateway: Ensure execution of correct document within multi-operation documents by including the operationName in the cache key used when caching query plans used in federated execution. [PR #3084](https://github-redirect.dependabot.com/apollographql/apollo-server/pull/3084)

• d0b33f2 Publish
• a6e14de Prepare for v2.7.2
• c2db83a Revert accidental change to package-lock.json in #3056
• ecbbc6a apollo-engine-reporting: fix reporting errors from backends (#3056)
• 31028e7 apollo-engine-reporting: clean up SIGINT/SIGTERM handlers on stop (#3090)
• 00c5274 Publish
• 62ef42a [Federation] Support @​extends directive for interfaces (#3083)
• c11a89a Update CHANGELOG.md post release with full version details.
• 2f87e4a Publish
• cdeb267 Update CHANGELOG.md prior to v2.7.1 release.
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