Async Clipboard: Rename tests and group tests in directories.

New directories (relative paths from //src/third_party/blink/web_tests/):
- `{clipboard/async-clipboard,external/wpt/clipboard-apis}/text-write-read`, to hold
text R/W tests.
- `external/wpt/clipboard-apis/events`, to hold tests ensuring firing
of copy/cut/paste events.
- 'clipboard/async-clipboard/permissions`, to hold tests testing for granted/denied
permissions properly allowing/denying a readText/writeText command.

File renames:
- Instead of `read-text` meaning a call to `navigator.clipboard.readText`, and
`read-blobtext` meaning a call to navigator.clipboard.write(ClipboardItem with text),
tests are now named as per function call names, for `readText` and `read`. Similar for
write functions.

Bug: 952650
Change-Id: I605577142a778f8aaaad38a947473cf21c962987
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1570707
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652010}