[wlkr73]

Adds a test beside test_find in Lib/test/test_str.py that drives the
adaptive search path with non-BMP characters.

Objects/stringlib/fastsearch.h compiles adaptive_find once per character
width, and only the ASCII and UCS1 copies ran under the whole suite:

Function 'ucs4lib_adaptive_find'   Lines executed:0.00% of 39
Function 'ucs2lib_adaptive_find'   Lines executed:66.67% of 39
Function 'ucs1lib_adaptive_find'   Lines executed:71.79% of 39
Function 'asciilib_adaptive_find'  Lines executed:79.49% of 39

The adaptive branch needs a haystack of at least 2500 characters, a needle of at
least 100, and a needle at least a third of the haystack. test_adaptive_find
meets all three with 'a' and 'b', so the UCS4 instantiation stayed cold.

The new test repeats that shape with '\U0001d11e' and '\U0001d11f', checking
both a miss and a hit, through find and count. With it,
ucs4lib_adaptive_find goes from 0.00% to 89.74% of 39 lines; the branches left
are the count and reverse-search modes, which are separate entry points and are
covered elsewhere.

Test-only change, no behaviour change, so no NEWS entry.

./python -m test test_str -m "*adaptive*" passes. The new test adds about
0.85 s to test_str on a -O0 --coverage build, which is slower than a normal
one; the whole module runs in roughly 14 s there.

[bedevere-app]

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[wlkr73]

No NEWS entry needed here: the guide lists "test changes" as exempt in "Changes that require NEWS entries" (devguide, pull-request-lifecycle). This PR only adds a test.