exercises: add acronym

[sudhackar]

Simple PR to add acronym exercise

$ ./bin/verify-exercises
acronym_test.c:83:test_basic:PASS
acronym_test.c:84:test_lowercase_words:PASS
acronym_test.c:85:test_punctuation:PASS
acronym_test.c:86:test_all_caps_word:PASS
acronym_test.c:87:test_punctuation_without_whitespace:PASS
acronym_test.c:88:test_very_long_abbreviation:PASS
acronym_test.c:89:test_consecutive_delimiters:PASS
acronym_test.c:90:test_apostrophes:PASS
acronym_test.c:91:test_underscore_emphasis:PASS

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[sudhackar]

In the test runner passes as well

91d5f8cf6d0c:/tmp/host# ./bin/verify-exercises acronym
acronym_test.c:83:test_basic:PASS
acronym_test.c:84:test_lowercase_words:PASS
acronym_test.c:85:test_punctuation:PASS
acronym_test.c:86:test_all_caps_word:PASS
acronym_test.c:87:test_punctuation_without_whitespace:PASS
acronym_test.c:88:test_very_long_abbreviation:PASS
acronym_test.c:89:test_consecutive_delimiters:PASS
acronym_test.c:90:test_apostrophes:PASS
acronym_test.c:91:test_underscore_emphasis:PASS

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9 Tests 0 Failures 0 Ignored
OK