std.Build: do not expect server protocol for tests using immature backends

For instance, when running a Zig test using the self-hosted aarch64
backend, this logic was previously expecting `std.zig.Server` to be
used, but the default test runner intentionally does not do this because
the backend is too immature to handle it. On 'master', this is causing
sporadic failures; on this branch, they became consistent failures.

lib/compiler/test_runner.zig

@@ -17,7 +17,9 @@ var fba_buffer: [8192]u8 = undefined;
 var stdin_buffer: [4096]u8 = undefined;
 var stdout_buffer: [4096]u8 = undefined;
 
-const crippled = switch (builtin.zig_backend) {
+/// Keep in sync with logic in `std.Build.addRunArtifact` which decides whether
+/// the test runner will communicate with the build runner via `std.zig.Server`.
+const need_simple = switch (builtin.zig_backend) {
     .stage2_aarch64,
     .stage2_powerpc,
     .stage2_riscv64,
@@ -33,7 +35,7 @@ pub fn main() void {
         return;
     }
 
-    if (crippled) {
+    if (need_simple) {
         return mainSimple() catch @panic("test failure\n");
     }
 
@@ -380,7 +382,7 @@ pub fn fuzz(
 
     // Some compiler backends are not capable of handling fuzz testing yet but
     // we still want CI test coverage enabled.
-    if (crippled) return;
+    if (need_simple) return;
 
     // Smoke test to ensure the test did not use conditional compilation to
     // contradict itself by making it not actually be a fuzz test when the test

lib/std/Build.zig

@@ -956,8 +956,37 @@ pub fn addRunArtifact(b: *Build, exe: *Step.Compile) *Step.Run {
             run_step.addArtifactArg(exe);
         }
 
-        const test_server_mode = if (exe.test_runner) |r| r.mode == .server else true;
-        if (test_server_mode) run_step.enableTestRunnerMode();
+        const test_server_mode: bool = s: {
+            if (exe.test_runner) |r| break :s r.mode == .server;
+            if (exe.use_llvm == false) {
+                // The default test runner does not use the server protocol if the selected backend
+                // is too immature to support it. Keep this logic in sync with `need_simple` in the
+                // default test runner implementation.
+                switch (exe.rootModuleTarget().cpu.arch) {
+                    // stage2_aarch64
+                    .aarch64,
+                    .aarch64_be,
+                    // stage2_powerpc
+                    .powerpc,
+                    .powerpcle,
+                    .powerpc64,
+                    .powerpc64le,
+                    // stage2_riscv64
+                    .riscv64,
+                    => break :s false,
+
+                    else => {},
+                }
+            }
+            break :s true;
+        };
+        if (test_server_mode) {
+            run_step.enableTestRunnerMode();
+        } else if (exe.test_runner == null) {
+            // If a test runner does not use the `std.zig.Server` protocol, it can instead
+            // communicate failure via its exit code.
+            run_step.expectExitCode(0);
+        }
     } else {
         run_step.addArtifactArg(exe);
     }