ARROW-15113: [C++] Make GcsFileSystem tests a bit faster

Before:

```
80/86 Test apache#50: arrow-gcsfs-test ..........................   Passed   13.52 sec
```

After:

```
64/86 Test apache#50: arrow-gcsfs-test ..........................   Passed    4.95 sec
```

Closes apache#11959 from coryan/ARROW-15113-gcsfs-speedup-tests-take2

Authored-by: Carlos O'Ryan <coryan@google.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/filesystem/gcsfs_test.cc

@@ -201,12 +201,13 @@ class GcsIntegrationTest : public ::testing::Test {
     const char* const kTestFolders[] = {
         "a/", "a/0/", "a/0/0/", "a/1/", "a/2/",
     };
+    constexpr auto kFilesPerFolder = 4;
     auto result = Hierarchy{PreexistingBucketPath() + "a/", {}};
     for (auto const* f : kTestFolders) {
       const auto folder = PreexistingBucketPath() + f;
       RETURN_NOT_OK(fs->CreateDir(folder, true));
       result.contents.push_back(arrow::fs::Dir(folder));
-      for (int i = 0; i != 64; ++i) {
+      for (int i = 0; i != kFilesPerFolder; ++i) {
         const auto filename = folder + "test-file-" + std::to_string(i);
         CreateFile(fs.get(), filename, filename);
         result.contents.push_back(arrow::fs::File(filename));