test: renable mutation tests

[rwalle61]

This renables mutation tests in the PR build, and restores the mutation score of chai-openapi-response-validator to nearly 100%.

The mutation tests were skipped while stryker's subdependency run-async's subdependency is-promise was broken. I've updated our version of run-async, which no longer uses is-promise

There are 4 mutants remaining. I'm happy to leave the first 3 because they are about error cases that I don't know how to test usefully. The 4th should be killed when we complete #57

153. [Survived] StringLiteral
/Users/[email protected]/Documents/Projects/SideProjects/OpenApiChai/OpenAPIValidators/packages/chai-openapi-response-validator/lib/openapi-validator/lib/classes/AbstractOpenApiSpec.js:138:8
-           'INVALID_OBJECT',
+           "",

178. [Survived] StringLiteral
/Users/[email protected]/Documents/Projects/SideProjects/OpenApiChai/OpenAPIValidators/packages/chai-openapi-response-validator/lib/openapi-validator/lib/classes/AbstractOpenApiSpec.js:107:8
-           'INVALID_BODY',
+           "",

180. [Survived] ConditionalExpression
/Users/[email protected]/Documents/Projects/SideProjects/OpenApiChai/OpenAPIValidators/packages/chai-openapi-response-validator/lib/openapi-validator/lib/classes/AbstractOpenApiSpec.js:90:10
-         if (error instanceof ValidationError) {
+         if (true) {

308. [Survived] ConditionalExpression
/Users/[email protected]/Documents/Projects/SideProjects/OpenApiChai/OpenAPIValidators/packages/chai-openapi-response-validator/lib/openapi-validator/lib/classes/OpenApi3Spec.js:73:8
-       if (!matchingServerBasePaths.length) {
+       if (false) {