.github: run tests on all PRs, regardless of branch name

The branch name selector "*" doesn't match branches with a "/" in their
name. The vast majority of our PRs are against the main (or previously,
master) branch anyway, so this will have minimal impact. But in the rare
cases that we want to open a PR against a branch with a "/" in the name,
tests should still run.

```
gh pr list --limit 9999 --state all --json baseRefName | \
  jq -cs '.[] | group_by(.baseRefName) |
    map({ base: .[0].baseRefName, count: map(.baseRefName) | length}) |
    sort_by(-.count) | .[]'

{"base":"main","count":4593}
{"base":"master","count":226}
{"base":"release-branch/1.48","count":4}
{"base":"josh-and-adrian-io_uring","count":3}
{"base":"release-branch/1.30","count":3}
{"base":"release-branch/1.32","count":3}
{"base":"release-branch/1.20","count":2}
{"base":"release-branch/1.26","count":2}
{"base":"release-branch/1.34","count":2}
{"base":"release-branch/1.38","count":2}
{"base":"Aadi/speedtest-tailscaled","count":1}
{"base":"josh/io_uring","count":1}
{"base":"maisem/hi","count":1}
{"base":"rel-144","count":1}
{"base":"release-branch/1.18","count":1}
{"base":"release-branch/1.2","count":1}
{"base":"release-branch/1.22","count":1}
{"base":"release-branch/1.24","count":1}
{"base":"release-branch/1.4","count":1}
{"base":"release-branch/1.46","count":1}
{"base":"release-branch/1.8","count":1}
{"base":"web-client-main","count":1}
```

Updates #cleanup

Signed-off-by: Will Norris <will@tailscale.com>

.github/workflows/test.yml

@@ -22,8 +22,7 @@ on:
       - "main"
       - "release-branch/*"
   pull_request:
-    branches:
-      - "*"
+    # all PRs on all branches
   merge_group:
     branches:
       - "main"