build: fix possible -Wformat-overflow in lib557 with test bundle builds

[vszakats]

• lib557: suppress -Wformat-overflow warning in source.
  Fixes:

  lib557.c: In function ‘test_float_formatting’:
  lib557.c:1408:37: error: ‘%*f’ directive output of 2147483648 bytes exceeds ‘INT_MAX’ [-Werror=format-overflow=]
   1408 |   curl_msnprintf(buf, sizeof(buf), "%*f", INT_MIN, 9.1);
        |                                     ^~~
  lib557.c:1408:3: note: ‘curl_msnprintf’ output 2147483649 bytes
   1408 |   curl_msnprintf(buf, sizeof(buf), "%*f", INT_MIN, 9.1);
        |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  

  Ref: https://app.circleci.com/pipelines/github/curl/curl/10226/workflows/87642ee9-cda6-4916-8206-c82aac5f595e/jobs/107669?invite=true#step-106-40996_46

  The root cause of why this option gets enabled remains undiscovered.

  Reported-by: Daniel Stenberg
  Fixes gcc-9 printf format warnings in lib557.c #15008
  Follow-up to 71cf0d1 tests: speed up builds with single-binary test bundles #14772

• build: drop -Wno-format-overflow from picky warning list.
  These options only get used with picky warnings enabled.
  Follow-up to 145f87b build: use -Wno-format-overflow #14598

• unit1652: suppress in source (and not rely on picky warnings anymore.)