Test `SHARABLE_MIDDLE_SUBSTRING` [ci skip]

.github/workflows/compilers.yml

@@ -179,6 +179,7 @@ jobs:
 #         - { name: USE_THREAD_CACHE=0,             env: { cppflags: '-DUSE_THREAD_CACHE=0' } }
           - { name: USE_RUBY_DEBUG_LOG=1,           env: { cppflags: '-DUSE_RUBY_DEBUG_LOG=1' } }
 #         - { name: USE_DEBUG_COUNTER,              env: { cppflags: '-DUSE_DEBUG_COUNTER=1', RUBY_DEBUG_COUNTER_DISABLE: '1' } }
+          - { name: SHARABLE_MIDDLE_SUBSTRING,      env: { cppflags: '-DSHARABLE_MIDDLE_SUBSTRING=1' } }
 
 #         - { name: DEBUG_FIND_TIME_NUMGUESS,       env: { cppflags: '-DDEBUG_FIND_TIME_NUMGUESS' } }
 #         - { name: DEBUG_INTEGER_PACK,             env: { cppflags: '-DDEBUG_INTEGER_PACK' } }