test(build): gate the maxConcurrent-1 test instead of racing a sleep

The build of branch-a slept one second while the test asserted, without
any synchronization, that branch-b was still PENDING.
Under CPU contention on the mih09 host the sleep could elapse first, so
branch-b was already RUNNING or SUCCESS when the assertion ran.
branch-a now blocks until the test creates a gate file, and the test
first waits for branch-a to be RUNNING; the PENDING assertion no longer
depends on timing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/test/kotlin/de/hoennig/werkator/build/BuildExecutorTest.kt

@@ -505,6 +505,8 @@ class BuildExecutorTest : FunSpec() {
         }
 
         test("with maxConcurrent 1 a second branch stays PENDING until the first finished") {
+            // branch-a blocks on a gate file the test creates, so the PENDING assertion
+            // below cannot race the first build finishing on a loaded machine
             val h =
                 Harness(
                     """
@@ -512,7 +514,7 @@ class BuildExecutorTest : FunSpec() {
                       maxConcurrent: 1
                     branches:
                       branch-a:
-                        buildCommand: "sleep 1"
+                        buildCommand: "until [ -f gate ]; do sleep 0.05; done"
                         cleanCommand: ""
                       branch-b:
                         buildCommand: "echo ok"
@@ -523,8 +525,13 @@ class BuildExecutorTest : FunSpec() {
             h.executor.startBuild(h.repo, "branch-a", "sha-a")
             h.executor.startBuild(h.repo, "branch-b", "sha-b")
 
+            eventually(30.seconds) {
+                h.repository.latestFor("branch-a")?.status shouldBe BuildStatus.RUNNING
+            }
             h.repository.latestFor("branch-b")?.status shouldBe BuildStatus.PENDING
 
+            Files.createFile(h.workingDir.resolve("gate"))
+
             awaitStatus(h, "branch-b", BuildStatus.SUCCESS)
             awaitStatus(h, "branch-a", BuildStatus.SUCCESS)
             val transitions = h.events.map { it.result.branch to it.result.status }