ARROW-14980: [C++] GCS tests use PYTHON environment variable

cpp/src/arrow/filesystem/gcsfs_test.cc

@@ -78,11 +78,32 @@ class GcsIntegrationTest : public ::testing::Test {
     // Initialize a PRNG with a small amount of entropy.
     generator_ = std::mt19937_64(std::random_device()());
     port_ = std::to_string(GetListenPort());
-    auto exe_path = bp::search_path("python3");
-    ASSERT_THAT(exe_path, Not(IsEmpty()));
-
-    server_process_ = bp::child(boost::this_process::environment(), exe_path, "-m",
-                                "testbench", "--port", port_, group_);
+    std::vector<std::string> names{"python3", "python"};
+    // If the build script or application developer provides a value in the PYTHON
+    // environment variable, then just use that.
+    if (const auto* env = std::getenv("PYTHON")) {
+      names = {env};
+    }
+    auto error = std::string(
+        "Cloud not start GCS emulator."
+        " Used the following list of python interpreter names:");
+    for (const auto& interpreter : names) {
+      auto exe_path = bp::search_path(interpreter);
+      error += " " + interpreter;
+      if (exe_path.empty()) {
+        error += " (exe not found)";
+        continue;
+      }
+
+      server_process_ = bp::child(boost::this_process::environment(), exe_path, "-m",
+                                  "testbench", "--port", port_, group_);
+      if (server_process_.valid() && server_process_.running()) break;
+      error += " (failed to start)";
+      server_process_.terminate();
+      server_process_.wait();
+    }
+    ASSERT_TRUE(server_process_.valid()) << error;
+    ASSERT_TRUE(server_process_.running()) << error;
 
     // Create a bucket and a small file in the testbench. This makes it easier to
     // bootstrap GcsFileSystem and its tests.