tests: compile-tests should only be executed in building machine

[cladmi]

Contribution description

When building with BUILD_IN_DOCKER the compile-tests were also tried to be executed on the host machine which had concurrency issues when done in parallel and also fails if you do not have the toolchain.

I also inlined added a change that verify the test file exists as the test would silently compare the error string instead of the real output.

Testing procedure

Using the first commit only:

On a clean environment these both fail:

BUILD_IN_DOCKER=1 DOCKER="sudo docker" BOARD=samr21-xpro make -C tests/cortexm_common_ldscript/ -j clean all
BUILD_IN_DOCKER=1 DOCKER="sudo docker" BOARD=samr21-xpro make -C tests/libc_newlib/ -j clean all

And with the following, the tests are executed both in docker and the build machine

BUILD_IN_DOCKER=1 DOCKER="sudo docker" BOARD=samr21-xpro make -C tests/libc_newlib/ all
...
Test: comparing addresses of 'printf' and 'iprintf' symbols
[SUCCESS] '0000158c' = '0000158c' is True
Test: comparing addresses of 'printf' and 'iprintf' symbols
[SUCCESS] '0000158c' = '0000158c' is True
make: Leaving directory '/home/harter/work/git/RIOT/tests/libc_newlib'

Issues/PRs references

Found while building in docker on a machine without toolchain and also again during #10344 testing.

[jcarrano]

Tested both commands, all good.

[cladmi]

I updated the test description as I pasted the same command twice for the first one.

Also I forgot to say that the fail case was with the first commit applied. Does not change what works, just how to show it failed.