tests/cpydiff: Ensure all have two levels of category.

This improves the TOC display of the generated differences section.

Signed-off-by: Jeff Epler <jepler@gmail.com>

tests/cpydiff/core_fstring_concat.py

@@ -1,5 +1,5 @@
 """
-categories: Core
+categories: Core,f-strings
 description: f-strings don't support concatenation with adjacent literals if the adjacent literals contain braces
 cause: MicroPython is optimised for code space.
 workaround: Use the + operator between literal strings when they are not both f-strings

tests/cpydiff/core_fstring_parser.py

@@ -1,5 +1,5 @@
 """
-categories: Core
+categories: Core,f-strings
 description: f-strings cannot support expressions that require parsing to resolve unbalanced nested braces and brackets
 cause: MicroPython is optimised for code space.
 workaround: Always use balanced braces and brackets in expressions inside f-strings

tests/cpydiff/core_fstring_repr.py

@@ -1,5 +1,5 @@
 """
-categories: Core
+categories: Core,f-strings
 description: f-strings don't support !a conversions
 cause: MicropPython does not implement ascii()
 workaround: None

tests/cpydiff/syntax_arg_unpacking.py

@@ -1,5 +1,5 @@
 """
-categories: Syntax
+categories: Syntax,Unpacking
 description: Argument unpacking does not work if the argument being unpacked is the nth or greater argument where n is the number of bits in an MP_SMALL_INT.
 cause: The implementation uses an MP_SMALL_INT to flag args that need to be unpacked.
 workaround: Use fewer arguments.

tests/cpydiff/syntax_spaces.py

@@ -1,5 +1,5 @@
 """
-categories: Syntax,Spaces
+categories: Syntax,Literals
 description: MicroPython requires spaces between literal numbers and keywords or ".", CPython doesn't
 cause: Different parser implementation