fix windows build and m_spec overflow

Note that a char[32] was too small. The test is implicitly testing this as well.
this test fails on my machine:

[ RUN      ] EngineFilterBiquadTest.fidlibInputRespectsLocale
src/test/enginefilterbiquadtest.cpp:27: Failure
Value of: spec
  Actual: "LsBq/1.2200000000/-12.0000000000"
Expected: "LsBq/1,2200000000/-12,0000000000"
[  FAILED  ] EngineFilterBiquadTest.fidlibInputRespectsLocale (1 ms)


notice the difference in decimal separator