http2_client is missing tests

[jeanbza]

The following cases are untested, and should be tested:

• grpc-go/internal/transport/http2_client.go

  Lines 275 to 278 in fb6867e

  	if err != nil {
  	t.Close()
  	return nil, connectionErrorf(true, err, "transport: failed to write client preface: %v", err)
  	}

• grpc-go/internal/transport/http2_client.go

  Lines 279 to 282 in fb6867e

  	if n != len(clientPreface) {
  	t.Close()
  	return nil, connectionErrorf(true, err, "transport: preface mismatch, wrote %d bytes; want %d", n, len(clientPreface))
  	}

• grpc-go/internal/transport/http2_client.go

  Lines 298 to 301 in fb6867e

  	if err != nil {
  	t.Close()
  	return nil, connectionErrorf(true, err, "transport: failed to write initial settings frame: %v", err)
  	}

• grpc-go/internal/transport/http2_client.go

  Lines 304 to 307 in fb6867e

  	if err := t.framer.fr.WriteWindowUpdate(0, delta); err != nil {
  	t.Close()
  	return nil, connectionErrorf(true, err, "transport: failed to write window update: %v", err)
  	}

e.g. changes in #2219 were almost merged that should not have because of lack of the aforementioned test coverage.

[v-sreejith]

@ginayeh I would like to contribute here

[arvindbr8]

Sure! Assigning it to you.

The original issue description might not be relevant anymore. I would re-evaluate the missed coverage of the file here.

[arvindbr8]

@v-sreejith -- ping.. Are you still actively tracking this?