Evaluate generator expressions in these properties, as they apply to
`add_test`, `add_custom_command`, and `add_custom_target`.
The `CMAKE_CROSSCOMPILING_EMULATOR` variable's `try_run` behavior occurs
at configure time and so cannot support generator expressions.
A `$<TARGET_FILE:tgt>` generator expression does not cause insertion of
the emulator. Add a test covering this. While at it, extend the test
case to cover executables in a subdirectory. Also make the test
matching expressions more robust to support multiple add_test calls
without mixing them while matching.
Prefix test commands with the CROSSCOMPILING_EMULATOR property
for target executables. This allows test suites to be run on the host
when crosscompiling.
