[test] OptionSetTest: fix elementary logic error

[lorentey]

As it turns out, if we move a mutating operation behind a conditional, then the effect of that mutation won't apply when the condition is false. 🤦‍♂🤦‍♀🤦‍♂🤦‍♀

rdar://57870367

[lorentey]

@swift-ci smoke test

[lorentey]

Hm; not sure what happened here:

13:14:03 Running Swift tests for: check-swift-validation-macosx-x86_64
13:14:03 /Users/buildnode/jenkins/workspace/swift-PR-osx-smoke-test/branch-master/swift/utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting
13:14:03 /Users/buildnode/jenkins/workspace/swift-PR-osx-smoke-test/branch-master/swift/utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting
13:14:03 Build step 'Execute shell' marked build as failure

[lorentey]

@swift-ci smoke test macOS platform

[lorentey]

13:16:53 Building remotely on macOS-66 (mac-pro) in workspace /Users/buildnode/jenkins/workspace/swift-PR-osx-smoke-test
13:16:53 No credentials specified
13:16:53  > git rev-parse --is-inside-work-tree # timeout=10
13:16:53 ERROR: Workspace has a .git repository, but it appears to be corrupt.
13:16:53 hudson.plugins.git.GitException: Command "git rev-parse --is-inside-work-tree" returned status code 137:

[lorentey]

@swift-ci smoke test macOS platform

[lorentey]

13:23:56 Compiler killed by signal 9

[xedin]

@swift-ci smoke test macOS platform