[CMake] download file, modify, add test from it

[Nico Schlömer]

Hi everyone,

I would like to create a CMake test that does the following:
 * Download a file from an external resource
 * Do something with the file
 * compile an executable myTest
 * execute myTest outputfile

Using ExternalData for downloading the file works well, but the second step
(file conversion, in this example cp for simplicity) does not work:
```
cmake_minimum_required(VERSION 3.2)

project(mytest)

# Download the files
INCLUDE(ExternalData)
set(
  ExternalData_URL_TEMPLATES
  "https://downloads.sourceforge.net/project/noshdata/%(algo)/%(hash)"
  )
ExternalData_Expand_Arguments(
  testFetchData
  OUT_DATA DATA{${CMAKE_SOURCE_DIR}/test.e}
  )
ExternalData_Add_Target(testFetchData)

add_custom_command(
  OUTPUT test.g
  COMMAND cp test.e test.g
  DEPENDS test.e
  )

ADD_EXECUTABLE("myTest" main.cpp)

add_test(myTest test.g)
```
I suppose I'm missing something about the dependencies here; after all, the
`add_custom_command` has to wait for `testFetchData` to complete. Likewise,
`add_test` has to wait for `add_custom_command` to complete.

Hints appreciated.

Cheers,
Nico
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