Make WebRequest::Get::Param unit test more forgiving

The simple test added in 59a8f26b01 (Add a unit test for wxWebRequest
query using URL parameters, 2021-03-06) worked when using httpbin.org,
but not when running httpbin locally, as it doesn't pretty-print JSON by
default.

Skip optional whitespace to make it work in both cases.

tests/net/webrequest.cpp

@@ -223,8 +223,21 @@ TEST_CASE_METHOD(RequestFixture,
 
     // We ought to really parse the returned JSON object, but to keep things as
     // simple as possible for now we just treat it as a string.
-    CHECK_THAT( request.GetResponse().AsString().ToStdString(),
-                Catch::Contains("\"pi\": \"3.14159265358979323\"") );
+    const wxString& response = request.GetResponse().AsString();
+    INFO("Response: " << response);
+
+    const char* expectedKey = "\"pi\":";
+    size_t pos = response.find(expectedKey);
+    REQUIRE( pos != wxString::npos );
+
+    pos += strlen(expectedKey);
+
+    // There may, or not, be a space after it.
+    while ( wxIsspace(response[pos]) )
+        pos++;
+
+    const char* expectedValue = "\"3.14159265358979323\"";
+    REQUIRE( response.compare(pos, strlen(expectedValue), expectedValue) == 0 );
 }
 
 TEST_CASE_METHOD(RequestFixture,