subprocess.Popen takes a cwd argument to set the Current Working Directory; you'll also want to escape your backslashes ('d:\\test\\local'), or use r'd:\test\local' so that the backslashes aren't interpreted as escape sequences by Python. The way you have it written, the \t part will be translated to a tab.
So, your new line should look like:
subprocess.Popen(r'c:\mytool\tool.exe', cwd=r'd:\test\local')
To use your Python script path as cwd, import os and define cwd using this:
os.path.dirname(os.path.realpath(__file__))