winmm: Use HeapAlloc in the capture test instead of using strdup.

[Dan Kegel]

Just like Maarten's patch in the wave test.

Fixes the valgrind error
http://kegel.com/wine/valgrind/logs-2008-05-25/vg-winmm_capture.txt
 Mismatched free() / delete / delete []
    at  RtlFreeHeap (heap.c:1287)
    by  wave_in_test_device (capture.c:397)
  Address 0x440cc90 is 0 bytes inside a block of size 14 alloc'd
    at  malloc (vg_replace_malloc.c:207)
    by  wave_in_test_device (capture.c:388)
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