Make the filesystem benchmark test less expensive (#7794)

While some of the slowness here is unrelated, there isn't really any
reason to test even as much of the benchmark as it is.

common/BUILD

@@ -287,9 +287,9 @@ sh_test(
     srcs = [":filesystem_benchmark"],
     args = [
         "--benchmark_dry_run",
-        # Restrict the sizes to 4-digit ones or smaller to keep test times low.
+        # Restrict the sizes to 2-digit ones or smaller to keep test times low.
         # The `$$` is repeated for Bazel escaping of `$`.
-        "--benchmark_filter=^[^/]+(/[0-9]{1,4}(/[0-9]+)?)?/real_time$$",
+        "--benchmark_filter=^[^/]+(/[0-9]{1,2}(/[0-9]+)?)?/real_time$$",
     ],
 )
 

common/filesystem_benchmark.cpp

@@ -444,9 +444,9 @@ auto BM_CreateDirectories(benchmark::State& state) -> void {
   CARBON_CHECK(existing_depth <= depth);
   CARBON_CHECK(depth > 0);
 
-  // Use a batch size of 10 to get avoid completely swamping the measurements
+  // Use a batch size of 5 to get avoid completely swamping the measurements
   // with overhead from creating existing directories and cleaning up.
-  constexpr int BatchSize = 10;
+  constexpr int BatchSize = 5;
 
   // Pre-build both the paths and the existing paths. Note that we use
   // relatively short paths here, which if anything makes the benefits of the