PSMDB-1880 Automatically set RUNFILES_DIR for unittests executed via resmoke

So far we have single test affected by this change: extensions_api_test
With this change we can succefully run it this way:

    buildscripts/resmoke.py run --suite unittests bazel-bin/install/bin/extensions_api_test

Author: igorsol

buildscripts/resmokelib/testing/testcases/cpp_unittest.py

@@ -1,5 +1,6 @@
 """The unittest.TestCase for C++ unit tests."""
 
+import os
 from typing import Optional
 
 from buildscripts.resmokelib import core, logging, utils
@@ -26,6 +27,44 @@ def __init__(
         self.program_options = utils.default_if_none(program_options, {}).copy()
 
         interface.append_process_tracking_options(self.program_options, self._id)
+        
+        # Automatically set RUNFILES_DIR for Bazel tests if not already set
+        self._set_runfiles_dir_if_needed()
+
+    def _set_runfiles_dir_if_needed(self):
+        """Set RUNFILES_DIR environment variable for Bazel tests if needed."""
+        # Check if RUNFILES_DIR is already set in program_options
+        env_vars = self.program_options.get("env_vars", {})
+        if "RUNFILES_DIR" in env_vars:
+            return  # Already set, don't override
+
+        # Check if this looks like a Bazel binary (in bazel-bin directory)
+        executable_path = os.path.abspath(self.program_executable)
+        if "bazel-bin" not in os.path.normpath(executable_path).split(os.sep):
+            return  # Not a Bazel binary, skip
+
+        # Check if executable path is a symlink and resolve it into resolved_path
+        resolved_path = executable_path
+        if os.path.islink(executable_path):
+            resolved_path = os.path.realpath(executable_path)
+
+        # Check if .runfiles directory exists next to the binary but take binary name from the symlink name
+        # Take base dir from resolved_path
+        base_dir = os.path.dirname(resolved_path)
+        # Take binary name from executable_path
+        binary_name = os.path.basename(executable_path)
+
+        runfiles_dir = os.path.join(base_dir, binary_name + ".runfiles")
+        if os.path.isdir(runfiles_dir):
+            # Set RUNFILES_DIR in env_vars
+            if "env_vars" not in self.program_options:
+                self.program_options["env_vars"] = {}
+            self.program_options["env_vars"]["RUNFILES_DIR"] = runfiles_dir
+            self.logger.debug(
+                "Automatically set RUNFILES_DIR=%s for Bazel test %s",
+                runfiles_dir,
+                executable_path,
+            )
 
     def _make_process(self):
         return core.programs.make_process(