./koch tests now does not rebuild nim

[timotheecour]

note to reviewer:
i also changed:

echo "Version check: failure"

to use a doAssert, as echo didn't seem to make sense

[Araq]

Merged it manually.

[timotheecour]

@Araq maybe I'm missing something obvious but c7c495f only sets var compilerPrefix* = "nim" ; I don't understand why you want to keep this in koch tests:

# Since tests take a long time (on my machine), and we want to defy Murhpys law - lets make sure the compiler really is freshly compiled!
nimexec "c --lib:lib -d:release --opt:speed compiler/nim.nim" # => which builds compiler/nim.exe

IIUC, the only place compiler/nim.exe was being used after this call was var compilerPrefix* = "compiler" / "nim" , which is why i had removed it from this PR