Having these named differently than the test itself mostly creates
unecessary confusion and makes writing logic against the tests harder
so let's rename the testsuite-xx units and scripts to just use the
test name itself.
Let's make this behave more like all the rest of the meson stuff.
This also is the first step to making it a bit more flexible so we
can define integration tests in different ways as will be seen in
the next commits.
Sometimes the ldconfig.service might take a bit longer to finish,
causing spurious test timeouts:
```
[ 1025.858923] systemd[24]: ldconfig.service: Executing: /sbin/ldconfig -X
...
[ 1043.883620] systemd[1]: ldconfig.service: Main process exited, code=exited, status=0/SUCCESS (success)
...
Trying to halt container. Send SIGTERM again to trigger immediate
termination.
Container TEST-52-HONORFIRSTSHUTDOWN terminated by signal KILL.
E: Test timed out after 20s
```
The $BUILD_DIR is only used in test-functions, and doesn't need to
be specified in any other scripts. Additionally, to be able to allow
the integration test suite to be run against locally installed binaries,
instead of built binaries, moving BUILD_DIR logic completely into
test-functions allows later patches to be simpler.
