feat: add phase 4 bun_test bootstrap

Co-authored-by: Eriyc <50216491+Eriyc@users.noreply.github.com>

internal/bun_test.bzl

@@ -1,2 +1,57 @@
-def bun_test(**_kwargs):
-    fail("bun_test is not implemented yet")
+"""Rule for running test suites with Bun."""
+
+
+def _bun_test_impl(ctx):
+    toolchain = ctx.toolchains["//bun:toolchain_type"]
+    bun_bin = toolchain.bun.bun_bin
+
+    src_args = " ".join(["\"{}\"".format(src.path) for src in ctx.files.srcs])
+    launcher = ctx.actions.declare_file(ctx.label.name)
+    ctx.actions.write(
+        output = launcher,
+        is_executable = True,
+        content = """#!/usr/bin/env bash
+set -euo pipefail
+
+extra_args=()
+if [[ -n "${{TESTBRIDGE_TEST_ONLY:-}}" ]]; then
+  extra_args+=("--test-name-pattern" "${{TESTBRIDGE_TEST_ONLY}}")
+fi
+if [[ -n "${{COVERAGE_DIR:-}}" ]]; then
+  extra_args+=("--coverage")
+fi
+
+exec "{}" test {} "${{extra_args[@]}}" "$@"
+""".format(bun_bin.path, src_args),
+    )
+
+    transitive_files = []
+    if ctx.attr.node_modules:
+        transitive_files.append(ctx.attr.node_modules[DefaultInfo].files)
+
+    runfiles = ctx.runfiles(
+        files = [bun_bin] + ctx.files.srcs + ctx.files.data,
+        transitive_files = depset(transitive = transitive_files),
+    )
+
+    return [
+        DefaultInfo(
+            executable = launcher,
+            runfiles = runfiles,
+        ),
+    ]
+
+
+bun_test = rule(
+    implementation = _bun_test_impl,
+    attrs = {
+        "srcs": attr.label_list(
+            mandatory = True,
+            allow_files = [".js", ".ts", ".jsx", ".tsx", ".mjs", ".cjs"],
+        ),
+        "node_modules": attr.label(),
+        "data": attr.label_list(allow_files = True),
+    },
+    test = True,
+    toolchains = ["//bun:toolchain_type"],
+)