add: refactor dynamic default tests to use xpath, extra test cases

- deleted tests where covered by case in TestDynamicDefaultSimpleInput
- updated tests using string matchers to use xpath instead
- added pyxform_test_case ability to escape literal pipe in values
- clarified existing translation performance test description
- added util function for coalesce
- added tests suggested by @lognaturel and performance tests
- updated test strategy for cases that initially or after rendering
  contain single quotes, namely to use xpath_exact to compare outside
  of xpath, instead of xpath_match.

pyxform/utils.py

@@ -390,6 +390,7 @@ def get_expression_lexer() -> re.Scanner:  # noqa
         "CLOSE_PAREN": r"\)",
         "BRACKET": r"\[\]\{\}",
         "PARENT_REF": r"\.\.",
+        "SELF_REF": r"\.",
         "PATH_SEP": r"\/",  # javarosa.xpath says "//" is an "unsupported construct".
         "SYSTEM_LITERAL": r""""[^"]*"|'[^']*'""",
         "COMMA": r",",
@@ -428,3 +429,7 @@ def parse_expression(text: str) -> Tuple[List[ExpLexerToken], str]:
     """
     tokens, remainder = EXPRESSION_LEXER.scan(text)
     return tokens, remainder
+
+
+def coalesce(*args):
+    return next((a for a in args if a is not None), None)

tests/pyxform_test_case.py

@@ -147,7 +147,8 @@ def assertPyxformXform(self, **kwargs):
         one of these possible survey input types
           * md: (str) a markdown formatted xlsform (easy to read in code)
                 [consider a plugin to help with formatting md tables,
-                 e.g. https://github.com/vkocubinsky/SublimeTableEditor]
+                 e.g. https://github.com/vkocubinsky/SublimeTableEditor].
+                Escape a literal pipe value with a single back-slash.
           * ss_structure: (dict) a python dictionary with sheets and their
                 contents. best used in cases where testing whitespace and
                 cells' type is important