gh-106320: _testcapi avoids private _PyUnicode_EqualToASCIIString()

[vstinner]

Replace private _PyUnicode_EqualToASCIIString() with public PyUnicode_CompareWithASCIIString().

• Issue: C API: Remove private C API functions (move them to the internal C API) #106320

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Arch Linux TraceRefs 3.x has failed when building commit 4e3aa7c.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/484/builds/3588) and take a look at the build logs.
4. Check if the failure is related to this commit (4e3aa7c) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/484/builds/3588

Failed tests:

• test__xxsubinterpreters

Summary of the results of the build (if available):

==

Click to see traceback logs

Note: switching to '4e3aa7cd312759922556ee96aa42033c375027e2'.

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Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at 4e3aa7cd31 gh-106320: _testcapi avoids private _PyUnicode_EqualToASCIIString() (#106341)
Switched to and reset branch 'main'

Objects/object.c:2197: _Py_ForgetReference: Assertion failed: invalid object chain
Enable tracemalloc to get the memory block allocation traceback

object address  : 0x7f10b2a1e200
object refcount : 0
object type     : 0x5565308eabc0
object type name: str
object repr     : <refcnt 0 at 0x7f10b2a1e200>

Fatal Python error: _PyObject_AssertFailed: _PyObject_AssertFailed
Python runtime state: initialized

Current thread 0x00007f10c4d82740 (most recent call first):
  <no Python frame>
Debug memory block at address p=0x7f10c4ca9ce0: API '�'
    18302063728033390045 bytes originally requested
    The 7 pad bytes at p-7 are not all FORBIDDENBYTE (0xfd):
        at p-7: 0xdd *** OUCH
        at p-6: 0xdd *** OUCH
        at p-5: 0xdd *** OUCH
        at p-4: 0xdd *** OUCH
        at p-3: 0xdd *** OUCH
        at p-2: 0xdd *** OUCH
        at p-1: 0xdd *** OUCH
    Because memory is corrupted at the start, the count of bytes requested
       may be bogus, and checking the trailing pad bytes may segfault.
    The 8 pad bytes at tail=0xfdfe7d0ec2c87abd are Fatal Python error: Segmentation fault

Current thread 0x00007f10c4d82740 (most recent call first):
  <no Python frame>

Extension modules: _testcapi, _xxsubinterpreters (total: 2)
make: *** [Makefile:2008: buildbottest] Segmentation fault (core dumped)

Cannot open file '/buildbot/buildarea/3.x.pablogsal-arch-x86_64/build/test-results.xml' for upload