Fix incorrect parsing logic in DawnTest

DawnTest incorrectly reports arguments as unused nor skips over ones
that were already parsed.

Change-Id: Ibd7874419226ca6404e44236c12fbd70132e4551
Bug: None
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/45442
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>

src/tests/DawnTest.cpp

@@ -258,6 +258,7 @@ void DawnTestEnvironment::ParseArgs(int argc, char** argv) {
             } else {
                 mBackendValidationLevel = dawn_native::BackendValidationLevel::Full;
             }
+            continue;
         }
 
         if (strcmp("-c", argv[i]) == 0 || strcmp("--begin-capture-on-startup", argv[i]) == 0) {
@@ -320,6 +321,7 @@ void DawnTestEnvironment::ParseArgs(int argc, char** argv) {
                     }
                 }
             }
+            continue;
         }
 
         constexpr const char kWireTraceDirArg[] = "--wire-trace-dir=";