* Get rid of libc_errno assignments in str_to_* __support tests, since
those API have been migrated to return error in a struct instead.
* Migrate tests for atof and to strto* functions from <stdlib.h> and for
strdup from <string.h> to use ErrnoCheckingTest harness.

Fixes #111546

---------

Co-authored-by: alyyelashram <150528548+alyyelashram@users.noreply.github.com>

Relates to
https://github.com/llvm/llvm-project/issues/119281#issuecomment-2699470459

This reverts commit 1e6e845d49a336e9da7ca6c576ec45c0b419b5f6 because it
changed the 1st parameter of adjust() to be unsigned, but libc itself
calls adjust() with a negative argument in align_backward() in
op_generic.h.

Relates to: #119281

docgen relies on the convention that we have a file foo.cpp in
libc/src/\<header\>/. Because the above functions weren't in libc/src/strings/
but rather libc/src/string/, docgen could not find that we had implemented
these.

Rather than add special carve outs to docgen, let's fix up our sources for
these 7 functions to stick with the existing conventions the rest of the
codebase follows.

Link: #118860
Fixes: #118875

The previous implementation of the ctype functions assumed ASCII.
This patch changes to a switch/case implementation that looks odd, but
actually is easier for the compiler to understand and optimize.

`strrchr("foo", '\0')` is defined to point to the end of `foo`, rather
than returning NULL. This wasn't caught by tests, since llvm-libc's
`ASSERT_STREQ(nullptr, "");` is not an assertion error.

While I'm here, refactor the test slightly to check for NULL more
specifically. I considered adding fancier `ASSERT`s (and changing the
semantics of `ASSERT_STREQ`), but opted for a more local fix by fair
dice roll.