[NFC][testing] Return underlying strings directly instead of OS.str()

This avoids an unnecessary copy required by 'return OS.str()', allowing
instead for NRVO or implicit move. The .str() call (which flushes the
stream) is no longer required since 65b13610,
which made raw_string_ostream unbuffered by default.

Differential Revision: https://reviews.llvm.org/D115374