Refactor (Simple)SpawnResult to reduce invalid representations

This moves status-related helper method implementations from the concrete
implementation to default methods on the interface, making their
semantics clearer, and encouraging other implementations of the interface
to behave the same.

This also strengthens status/exitCode assertions in SpawnResult.Builder's
build method. This makes misuse less likely.

RELNOTES: None.
PiperOrigin-RevId: 296071684

src/test/java/com/google/devtools/build/lib/actions/SpawnResultTest.java

diff --git a/src/test/java/com/google/devtools/build/lib/actions/SpawnResultTest.java b/src/test/java/com/google/devtools/build/lib/actions/SpawnResultTest.java
index 59de66f..2e9b0ff 100644
--- a/src/test/java/com/google/devtools/build/lib/actions/SpawnResultTest.java
+++ b/src/test/java/com/google/devtools/build/lib/actions/SpawnResultTest.java
@@ -38,7 +38,7 @@
         new SpawnResult.Builder()
             .setStatus(SpawnResult.Status.TIMEOUT)
             .setWallTime(Duration.ofSeconds(5))
-            .setExitCode(1)
+            .setExitCode(SpawnResult.POSIX_TIMEOUT_EXIT_CODE)
             .setRunnerName("test")
             .build();
     assertThat(r.getDetailMessage("", "", false, false, false))
@@ -50,7 +50,7 @@
     SpawnResult r =
         new SpawnResult.Builder()
             .setStatus(SpawnResult.Status.TIMEOUT)
-            .setExitCode(1)
+            .setExitCode(SpawnResult.POSIX_TIMEOUT_EXIT_CODE)
             .setRunnerName("test")
             .build();
     assertThat(r.getDetailMessage("", "", false, false, false))