run-tests: Make .exp and .out file names unique by prefixing with dir.

Input files like basics/string_format.py and float/string_format.py have
the same basename so using that name for writing the output (.exp and .out
files) when both tests fail, results in the output of the first one being
overwritten.

Avoid this by using unique names for the output, replacing path characters
with underscores.

tests/run-tests

@@ -394,8 +394,8 @@ def run_tests(pyb, tests, args, base_path="."):
             if verdict == "exclude":
                 continue
 
-        test_basename = os.path.basename(test_file)
-        test_name = os.path.splitext(test_basename)[0]
+        test_basename = test_file.replace('..', '_').replace('./', '').replace('/', '_')
+        test_name = os.path.splitext(os.path.basename(test_file))[0]
         is_native = test_name.startswith("native_") or test_name.startswith("viper_")
         is_endian = test_name.endswith("_endian")
         is_int_big = test_name.startswith("int_big") or test_name.endswith("_intbig")