Move Content sandbox test support into Content

This code was originally landed when sandbox stuff was still in Service
Manager. It's not in Service Manager anymore, and Service Manager code
is on its way to the great Git repository in the sky.

Bug: 977637
Change-Id: I10c2c02cbd70bc3f99fb0dfdb26319003b0047c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520317
Auto-Submit: Ken Rockot <rockot@google.com>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824301}

• content/browser/back_forward_cache_browsertest.cc[diff]
• content/browser/renderer_host/frame_service_base_unittest.cc[diff]
• content/browser/utility_process_sandbox_browsertest.cc[diff]
• content/shell/BUILD.gn[diff]
• content/shell/utility/shell_content_utility_client.cc[diff]
• content/test/BUILD.gn[diff]
• content/test/echo.test-mojom[Renamed from content/test/echo.mojom - diff]
• content/test/sandbox_status.test-mojom[Renamed from services/service_manager/tests/sandbox_status.test-mojom - diff]
• content/test/sandbox_status_service.cc[Renamed from services/service_manager/tests/sandbox_status_service.cc - diff]
• content/test/sandbox_status_service.h[Renamed from services/service_manager/tests/sandbox_status_service.h - diff]
• services/service_manager/tests/BUILD.gn[diff]