tests/run-tests.py: Make skip logic work irregardless of test path.

Test file paths which get passed to the run_tests function can be
absolute or relative and with or without leading slash in the latter
case, depending on the arguments to run-tests.py, but the skip_tests
list with tests to skip only contains relative paths so using simple
string equality comparison easily leads to false negatives.

Compare the full absolute path instead such that it doesn't matter
anymore in which form the tests are passed. Note:
- use realpath to resolve symlinks plus make the comparison case
  insensitive on windows
- the test_file passed to run_one_test is not altered by this commit,
  such that when the user inputs relative paths the tests are also
  still displayed with relative paths
- likewise the test_file_abspath is not modified because functions
  like run_micropython rely on it having forward slashes

In practice this means that it used to be so that the only forms
of running tests for which the skip_tests lists actually worked were:

 >python ./run-tests.py
 >python ./run-tests.py -d extmod

whereas it now works consistently so also for these invocations,
which in the end all point to the exact same path:

 >python ./run-tests.py -d ./extmod
 >python ./run-tests.py -d ../tests/extmod
 >python ./run-tests.py -d /full/path/to/tests/extmod

These examples used to not skip any of the tests in the extmod/
directory thereby leading to test failures.

Signed-off-by: stijn <stijn@ignitron.net>

tests/run-tests.py

@@ -838,6 +838,8 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
             # Works but CPython uses '\' path separator
             skip_tests.add("import/import_file.py")
 
+    skip_tests = [os.path.realpath(base_path(skip_test)) for skip_test in skip_tests]
+
     def run_one_test(test_file):
         test_file = test_file.replace("\\", "/")
         test_file_abspath = os.path.abspath(test_file).replace("\\", "/")
@@ -869,7 +871,7 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
         is_fstring = test_name.startswith("string_fstring")
         is_inlineasm = test_name.startswith("asm")
 
-        skip_it = test_file in skip_tests
+        skip_it = os.path.realpath(test_file) in skip_tests
         skip_it |= skip_native and is_native
         skip_it |= skip_endian and is_endian
         skip_it |= skip_int_big and is_int_big