test(external): tests that share a dependency fetch it once a run

Eight tests that include a header from the same tracked branch of a
local repository go through RunTests at once, and again after the
branch has moved. GIT_TRACE logs every git process, and each fetch of
the repository, clone or pull, runs one `git upload-pack` in it, so the
trace counts the fetches: one per run rather than one per test, and the
second run still pulls and brings the clone to the new commit.

On the commit before the fix both runs fetch eight times. The tests
themselves pass there, because the lock from 653dd0b already keeps the
pulls apart. Going through RunTests, rather than Build() on threads of
the test's own, also holds RunTests to building its tests with one
depResults.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

project.cpp

@@ -262,6 +262,7 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         cfg.AddTest("LinkFlagsChange").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ExternalArtifactTime").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("ConcurrentExternalFetch").Dependencies({ CrafterBuildLib.get() });
+        cfg.AddTest("SharedExternalFetch").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("StandardArgs").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("TestRunnerSpec").Dependencies({ CrafterBuildLib.get() });
         cfg.AddTest("VariantId").Dependencies({ CrafterBuildLib.get() });