Fix flaky unittest failures (#7544)

Fix three unit tests which have been flakily failing for the last
several weeks:

//test/load-generator/acme: TestNew/unreachable_directory_URL
Fixed by changing the error checking code to care only about the
underlying "connection refused" message, and not the IP address from
which it was receieved.

//va: TestHTTPDialTimeout
Fixed by correcting the error checking code to look for "network is
unreachable" instead of "Network unreachable"

//va: TestFetchHTTP/Broken_IPv6_only
Fixed by making the expected error message more specific -- it was
previously looking for "Error getting validation data", which is the
message that `detailedError` gives for errors it doesn't recognize. An
underlying library has changed to provide an error type that
`detailedError` now recognizes as a connection error.

test/load-generator/acme/directory_test.go

@@ -132,7 +132,7 @@ func TestNew(t *testing.T) {
 		{
 			Name:          "unreachable directory URL",
 			DirectoryURL:  "http://localhost:1987",
-			ExpectedError: "Get \"http://localhost:1987\": dial tcp 127.0.0.1:1987: connect: connection refused",
+			ExpectedError: "connect: connection refused",
 		},
 		{
 			Name:          "wrong directory HTTP status code",
@@ -179,7 +179,7 @@ func TestNew(t *testing.T) {
 			if err == nil && tc.ExpectedError != "" {
 				t.Errorf("expected error %q got nil", tc.ExpectedError)
 			} else if err != nil {
-				test.AssertEquals(t, err.Error(), tc.ExpectedError)
+				test.AssertContains(t, err.Error(), tc.ExpectedError)
 			}
 		})
 	}

va/http_test.go

@@ -992,7 +992,7 @@ func TestFetchHTTP(t *testing.T) {
 			Host: "ipv6.localhost",
 			Path: "/ok",
 			ExpectedProblem: probs.Connection(
-				"::1: Fetching http://ipv6.localhost/ok: Error getting validation data"),
+				"::1: Fetching http://ipv6.localhost/ok: Connection refused"),
 			ExpectedRecords: []core.ValidationRecord{
 				{
 					Hostname:          "ipv6.localhost",
@@ -1098,14 +1098,14 @@ func TestFetchHTTP(t *testing.T) {
 			ctx, cancel := context.WithTimeout(context.Background(), time.Millisecond*500)
 			defer cancel()
 			body, records, err := va.fetchHTTP(ctx, tc.Host, tc.Path)
-			if err != nil && tc.ExpectedProblem == nil {
+			if tc.ExpectedProblem == nil {
-				t.Errorf("expected nil prob, got %#v\n", err)
+				test.AssertNotError(t, err, "expected nil prob")
-			} else if err == nil && tc.ExpectedProblem != nil {
+			} else {
-				t.Errorf("expected %#v prob, got nil", tc.ExpectedProblem)
+				test.AssertError(t, err, "expected non-nil prob")
-			} else if err != nil && tc.ExpectedProblem != nil {
 				prob := detailedError(err)
 				test.AssertMarshaledEquals(t, prob, tc.ExpectedProblem)
-			} else {
+			}
+			if tc.ExpectedBody != "" {
 				test.AssertEquals(t, string(body), tc.ExpectedBody)
 			}
 			// in all cases we expect validation records to be present and matching expected
@@ -1378,7 +1378,7 @@ func TestHTTPDialTimeout(t *testing.T) {
 	var err error
 	for range 20 {
 		_, err = va.validateHTTP01(ctx, dnsi("unroutable.invalid"), expectedToken, expectedKeyAuthorization)
-		if err != nil && strings.Contains(err.Error(), "Network unreachable") {
+		if err != nil && strings.Contains(err.Error(), "network is unreachable") {
 			continue
 		} else {
 			break