Same idea as a7be4b7bf857, but that only showed the context if the
length of the string differed. Since these tests check both string
length and string content, the ctx should be provided for both.

    ERROR    test/unit/testutil.lua @ 797: vim_snprintf() positional arguments
    test/unit/testutil.lua:769: test/unit/testutil.lua:753: (string) '
    test/unit/strings_spec.lua:159: snprintf(buf, 4, "%1$0.*2$b", 12ULL, cdata<int>: 0xf78c8ed8) = 001100
    Expected objects to be the same.
    Passed in:
    (string) '000'
    Expected:
    (string) '001''

Problem: On 32-bit architectures, musl libc makes heavy use of
__typeof__ as part of its __REDIR macro for optional backwards
compatibility with 32-bit time_t values. Unfortunately, the
__typeof__ keyword is not supported by the LuaJIT C parser.

Solution: Filter out the keyword in filter_complex_blocks.

Problem: This keyword is used by GCC and Clang to prevent -Wpedantic
(and other options) from emitting warnings for many GNU C extensions.
This is used heavily in Alpine Linux through musl libc and
foritfy-headers. Without filtering the __extension__ keyword some type
definitions are duplicated. For example, timeval is defined once as

  struct timeval { time_t tv_sec; suseconds_t tv_usec; };

and once as:

  __extension__ struct timeval { time_t tv_sec; suseconds_t tv_usec; };

Without this patch, the LuaJIT C parser doesn't recognize that these
definitions are equivalent, causing unit test to fail on Alpine Linux.

Solution: Filter out the keyword in filter_complex_blocks.

When 9.0.1856 was ported, the numbers being formatted as binary were cast
to "unsigned int" rather than uvarnumber_T, as is done upstream.

Since these assertions all use a common function to perform the test
assertions, it's difficult to figure out which test failed:

    ERROR    test/unit/testutil.lua @ 785: vim_snprintf() positional arguments
    test/unit/testutil.lua:757: test/unit/testutil.lua:741: (string) '
    test/unit/strings_spec.lua:143: Expected objects to be the same.
    Passed in:
    (number) 6400
    Expected:
    (number) 6'
    exit code: 256

Adding context to the assertion makes it clearer what the problem is:

    ERROR    test/unit/testutil.lua @ 785: vim_snprintf() positional arguments
    test/unit/testutil.lua:757: test/unit/testutil.lua:741: (string) '
    test/unit/strings_spec.lua:149: snprintf(buf, 0, "%1$0.*2$b", cdata<unsigned int>: 0xf78d0f38, cdata<int>: 0xf78dc4e0) = 001100
    Expected objects to be the same.
    Passed in:
    (number) 6400
    Expected:
    (number) 6'
    exit code: 256

Ref:
https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h3-Other-buttons