ci(fix): Fix flaky macOS tests (#11128)

### Description

Some tests are flaky on macOS. Choosing to run them serially to see if
they stop flaking.

This likely only masks a root cause(s) that we should investigate
further in the future.

### Testing Instructions

CI

<sub>CLOSES TURBO-4963</sub>

Author: anthonyshew

.config/nextest.toml

@@ -0,0 +1,22 @@
+# Nextest configuration for Turborepo
+# See https://nexte.st/book/configuration.html
+
+[test-groups]
+# turborepo-process tests should run serially to avoid resource contention
+# and timing issues, especially on macOS where PTY devices are limited
+turborepo-process-serial = { max-threads = 1 }
+
+# Integration tests that spawn turbo binaries should run serially to avoid
+# race conditions with binary detection and stdout/stderr capture
+turborepo-integration-serial = { max-threads = 1 }
+
+[[profile.default.overrides]]
+# Run all tests in the turborepo-process crate serially
+filter = 'package(turborepo-process)'
+test-group = 'turborepo-process-serial'
+
+[[profile.default.overrides]]
+# Run integration tests that use check_json_output! serially
+# These tests spawn turbo processes and parse JSON from stdout
+filter = 'package(turbo) and (test(boundaries) or test(query) or test(ls))'
+test-group = 'turborepo-integration-serial'