test: decouple test case from typescript version

[ericcrosson-bitgo]

The externalModuleApiSpec tests are currently using the installed
TypeScript version as the api spec version. This means when we bump the
TypeScript version, we break tests.

This commit specifies a literal version number instead of pulling the
version number from TypeScript, so tests don't break when we update the
TypeScript version.

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🎉 This PR is included in version @api-ts/openapi-generator@5.7.0 🎉

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🎉 This PR is included in version @api-ts/typed-express-router@2.0.0-beta.1 🎉

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🎉 This PR is included in version @api-ts/express-wrapper@2.0.0-beta.1 🎉

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🎉 This PR is included in version @api-ts/typed-express-router@1.1.14 🎉

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🎉 This PR is included in version @api-ts/express-wrapper@1.0.34 🎉

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🎉 This PR is included in version @api-ts/superagent-wrapper@1.3.4 🎉

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