crypto/tls: pretty-print SignatureScheme in tests

When running crypto/tls tests with GOEXPERIMENT=boringcrypto, some
tests are embedded with unreadable hexadecimal values:

  === RUN   TestBoringServerSignatureAndHash/5053...3536

This corresponds to a string representation of SignatureScheme as it
implements fmt.Stringer.  With this change, the above will be printed
as:

  === RUN   TestBoringServerSignatureAndHash/PSSWithSHA256

Change-Id: I953c0bb35c68e77a7f01e7f1fceda203c272faf7
GitHub-Last-Rev: 19700d53a8578d335dc803ac94cc7c6c72e9920a
GitHub-Pull-Request: golang/go#63175
Reviewed-on: https://go-review.googlesource.com/c/go/+/530715
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>

tls/boring_test.go

@@ -200,7 +200,7 @@ func TestBoringServerSignatureAndHash(t *testing.T) {
 	}()
 
 	for _, sigHash := range defaultSupportedSignatureAlgorithms {
-		t.Run(fmt.Sprintf("%#x", sigHash), func(t *testing.T) {
+		t.Run(fmt.Sprintf("%v", sigHash), func(t *testing.T) {
 			serverConfig := testConfig.Clone()
 			serverConfig.Certificates = make([]Certificate, 1)