build: add test subcommand

zig build test can now run tests defined in main.zig and all files it
imports due to std.testing.refAllDecls.

build.zig

@@ -17,7 +17,12 @@ pub fn build(b: *std.build.Builder) void {
     if (b.args) |args| {
         run_cmd.addArgs(args);
     }
-
     const run_step = b.step("run", "run the executable");
     run_step.dependOn(&run_cmd.step);
+
+    const exe_tests = b.addTest("src/main.zig");
+    exe_tests.setTarget(target);
+    exe_tests.setBuildMode(bmode);
+    const test_step = b.step("test", "run tests");
+    test_step.dependOn(&exe_tests.step);
 }

src/main.zig

@@ -176,3 +176,8 @@ fn zigStdPath(alloc: std.mem.Allocator) ![]const u8 {
     defer std.json.parseFree(Env, jenv, opt);
     return alloc.dupe(u8, jenv.std_dir);
 }
+
+test {
+    // run tests found in all @import'ed files.
+    std.testing.refAllDecls(@This());
+}