Changes

tests/testsuite.at: fix invalid if-endif syntax

`make check` currently fails if libosmocore is configured with
`--disable-msgfile`.  The reason is that the testsuite tries to
execte a binary that does not exist.  This is happening because
the if-endif syntax is invalid and thus ignored.

Change-Id: I81e044dc0bda4674c0d0dc46118d46816712a76c

tests/testsuite.at: use AT_SKIP_IF for sim_test

Whether to run the given test or not can be determined by presence
of the respective `*_test` binary, since we build them conditionally
in the `Makefile.am`.  Get rid of more complicated logic by using
AT_SKIP_IF and checking presence of `tests/sim/sim_test`.

Change-Id: Ic90b3a39936888eda6c9d443420c4faa131ac78f

tests/testsuite.at: properly skip disabled tests

We should not be running these tests unconditionally because the
respective binaries are built conditionally and may be missing.

Change-Id: I7e9a0c84e510964e5d577e371d41f2023ab20840