ci: ignore tests in single feature build

[dbolduc]

Part of the work for #8022

The single-feature build is only concerned with verifying installed targets. It does not need to build the testing targets.

This matters, because tests in grpc_utils depend on some libraries protos (bigtable_protos, iam_protos, spanner_protos) and we are no longer going to compile these protos by default, unless those features are enabled.

For the build failure we are avoiding, see:

• GCB: https://console.cloud.google.com/cloud-build/builds;region=us-east1/224eb02b-5763-4638-8cfe-7232943dba5c;tab=detail?project=cloud-cpp-testing-resources
• Raw: https://storage.googleapis.com/cloud-cpp-community-publiclogs/logs/google-cloud-cpp/12462/96c2c14c65e71a542e5bae5c3c4d5ea2725f8ccd/fedora-37-cmake-cmake-single-feature/log-224eb02b-5763-4638-8cfe-7232943dba5c.txt

---

This change is 

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (af7e22c) 93.62% compared to head (bd6a364) 93.61%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12464      +/-   ##
==========================================
- Coverage   93.62%   93.61%   -0.01%     
==========================================
  Files        2039     2039              
  Lines      180676   180676              
==========================================
- Hits       169153   169148       -5     
- Misses      11523    11528       +5     

see 6 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[scotthart]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dbolduc)