internal/wycheproof: skip on builders with flaky network connections

This test downloads JSON test files from GitHub. If the network
connection flakes, the test may hang indefinitely, and builders with
flaky network connections might want to avoid unnecessary downloading
anyway.

Fixes golang/go#50076

Change-Id: I0655a34fce8a003794290c0c82ae75492eda4429
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/370659
Trust: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Katie Hockman <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: Bryan C. Mills

internal/wycheproof/wycheproof_test.go

@@ -33,6 +33,10 @@ func TestMain(m *testing.M) {
 		log.Printf("skipping test because 'go' command is unavailable: %v", err)
 		os.Exit(0)
 	}
+	if os.Getenv("GO_BUILDER_FLAKY_NET") != "" {
+		log.Printf("skipping test because GO_BUILDER_FLAKY_NET is set")
+		os.Exit(0)
+	}
 
 	// Download the JSON test files from github.com/google/wycheproof
 	// using `go mod download -json` so the cached source of the testdata