Deflake limit_test.
Deflake limit_test.

* This avoids allocating buffers for statistics from malloc while we are under
  a limit.
* For the expected string, use absl::SNPrintF to build up in a stack-allocated
  buffer.
* gmock's matchers for strings take a const std::string&, requiring an
  allocation.  Avoid this by comparing the absl::string_view instances
  directly.

PiperOrigin-RevId: 314209546
Change-Id: I9b0cbf75714a87831a49a029b365610763bb1e78

google/tcmalloc,google/tcmalloc,google/tcmalloc