CMake: Ensure MARL_BUILD_TESTS works

[awilfox]

The conditional for building tests was written as simply "BUILD_TESTS".
This commit changes it to use "MARL_BUILD_TESTS".

With this commit, my POWER9 in Big Endian mode can build, run, and pass
the Marl test suite.

[googlebot]

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[awilfox]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[awilfox]

Who would I poke about an issue with googlebot? There's a stray close-paren in the directive to respond once the CLA has been signed.

[ben-clayton]

Thank you @awilfox. I must have broken this earlier today.
I'm currently trying to get our presubmits running which should catch this sort of thing in the future.
Very much appreciate the fix!

[ben-clayton]

Who would I poke about an issue with googlebot? There's a stray close-paren in the directive to respond once the CLA has been signed.

Thanks for also spotting and reporting this! I'll do some digging and see if I can find the right people.

[ben-clayton]

Who would I poke about an issue with googlebot? There's a stray close-paren in the directive to respond once the CLA has been signed.

I reported this to the relevant team, and they have made a fix. It should take affect sometime next week.

Thank you!