Fixed the clang complation problem for the executor tests.
Fixed the clang complation problem for the executor tests.

It appears that g++ throws away the cv-qualifiers when doing the
lvalue-to-rvalue conversion for lambdas returning strings but clang does
not, so we make `f3` in `THREADSAFE_Executor_Execute` return a
non-constant string.

Review: https://reviews.apache.org/r/62698/

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