tsdb/chunks: preallocate at least some space on non-Windows systems
To avoid potential chunk corruption read, which I am not sure why is
happening.
Closes
package chunks
// HeadChunkFilePreallocationSize is the size to which the m-map file should be preallocated when a new file is cut.
-// Windows needs pre-allocations while the other OS does not.
-var HeadChunkFilePreallocationSize int64
+// Windows needs pre-allocations while the other OS does not. But we observed that a 0 pre-allocation causes unit tests to flake.
+// This small allocation for non-Windows OSes removes the flake.
+var HeadChunkFilePreallocationSize int64 = MinWriteBufferSize * 2
