Update "LayoutTests" / "layout test" in docs/*.md.

This CL renames the following md files:
 - layout_tests_linux -> web_tests_linux.md
 - how_to_extend_layout_test_framework.md ->
   how_to_extend_web_test_framework.md
 - testing/layout_test_baseline_fallback.md ->
   testing/web_test_baseline_fallback.md
 - testing/layout_test_expectations.md -> testing/web_test_expectations.md
 - testing/layout_tests.md -> testing/web_tests.md
 - testing/layout_tests_in_content_shell.md ->
   testing/web_tests_in_content_shell.md
 - testing/layout_tests_tips.md -> testing/web_tests_tips.md
 - testing/layout_tests_with_manual_fallback.md ->
   testing/web_tests_with_manual_fallback.md
 - testing/writing_layout_tests -> testing/writing_web_tests.md

No-Try: true
Change-Id: Ied34815c92e449888b20818540a5ed0a76107bd1
Bug: 843412
Reviewed-on: https://chromium-review.googlesource.com/c/1350559
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611024}
41 files changed