Fix GetTextString() and GetByteString() error handling (#303)

GetTextString() and GetByteString() where returning the wrong error code when the input CBOR was the wrong type

* Fix GetTextString() and GetByteString() error handling

* Improve test and doc for GetString

* Miniscule header fix

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/test/run_tests.c b/test/run_tests.c
index bcb4540..f186f57 100644
--- a/test/run_tests.c
+++ b/test/run_tests.c
@@ -131,7 +131,7 @@
    TEST_ENTRY(MemPoolTest),
    TEST_ENTRY(IndefiniteLengthStringTest),
 #ifndef QCBOR_DISABLE_NON_INTEGER_LABELS
-   TEST_ENTRY(SpiffyIndefiniteLengthStringsTests),
+   TEST_ENTRY(SpiffyStringTest),
 #endif /* ! QCBOR_DISABLE_NON_INTEGER_LABELS */
    TEST_ENTRY(SetUpAllocatorTest),
    TEST_ENTRY(CBORTestIssue134),