build test: install unit tests binary

I've gotten tired of trying to find this in the cache dir, since the
build system does not rerun it once it is cached. Also sometimes the
build runner does weird things to the output.

build.zig

@@ -19,6 +19,7 @@ pub fn build(b: *std.Build) void {
     });
 
     const tests = b.addTest(.{
+        .name = "nats-zig-unit-tests",
         .root_source_file = .{ .path = "tests/main.zig" },
         .target = target,
         .optimize = optimize,
@@ -29,6 +30,7 @@ pub fn build(b: *std.Build) void {
 
     const run_main_tests = b.addRunArtifact(tests);
     const test_step = b.step("test", "Run tests");
+    test_step.dependOn(&b.addInstallArtifact(tests, .{}).step);
     test_step.dependOn(&run_main_tests.step);
 
     add_examples(b, .{