test: use ${builddir}/systemd-runtest.env to set $SYSTEMD_TEST_DATA

test: use ${builddir}/systemd-runtest.env to set $SYSTEMD_TEST_DATA

This simplifies get_testdata_dir() to simply checking for an environment
variable, with an additional function to locate a systemd-runtest.env file in
the same directory as the test binary and reading environment variable
assignments from that file if it exists.

This makes it possible to:
- Run `ninja test` from the build dir and have it use ${srcdir}/test for
  test unit definitions.
- Run a test directly, such as `build/test-execute` and have it locate
  them correctly.
- Run installed tests (from systemd-tests package) and locate the test
  units in the installed location (/usr/lib/systemd/tests/testdata), in
  which case the absence of the systemd-runtest.env file will have
  get_testdata_dir() use the installed location hardcoded into the
  binaries.

Explicit setting of $SYSTEMD_TEST_DATA still overrides the contents of
systemd-runtest.env.