cmd/testwrapper: parse args better

Previously we were just smushing together args and not trying
to parse the values at all. This resulted in the args to testwrapper
being limited and confusing.

This makes it so that testwrapper parses flags in the exact format as `go test`
command and passes them down in the provided order. It uses tesing.Init to
register flags that `go test` understands, however those are not the only
flags understood by `go test` (such as `-exec`) so we register these separately.

Updates tailscale/corp#14975

Signed-off-by: Maisem Ali <maisem@tailscale.com>

.github/workflows/test.yml

@@ -47,7 +47,7 @@ jobs:
     - name: build test wrapper
       run: ./tool/go build -o /tmp/testwrapper ./cmd/testwrapper
     - name: integration tests as root
-      run: PATH=$PWD/tool:$PATH /tmp/testwrapper --sudo ./tstest/integration/ -race
+      run: PATH=$PWD/tool:$PATH /tmp/testwrapper -exec "sudo -E" -race ./tstest/integration/
 
   test:
     strategy: