system/irtest/Make.defs: fixed the wrong path

[simbit18]

Summary

fix this error
make[3] *** /nuttxspace/nuttx/apps/testing/irtest: No such file or directory. Stop.

same error with macOS on Nuttx mirror

make: *** /Users/runner/work/nuttx/nuttx/sources/apps/testing/irtest: No such file or directory.  Stop.
make[2]: *** [/Users/runner/work/nuttx/nuttx/sources/apps/testing/irtest_context] Error 2
make[2]: Target `context_all' not remade because of errors.
make[1]: *** [context] Error 2
make: *** [/Users/runner/work/nuttx/nuttx/sources/apps/.context] Error 2
make: Target `all' not remade because of errors.
/Users/runner/work/nuttx/nuttx/sources/nuttx/tools/testbuild.sh: line 385: /Users/runner/work/nuttx/nuttx/sources/nuttx/../nuttx/nuttx.manifest: No such file or directory

https://github.com/NuttX/nuttx/actions/runs/13430411340/job/37521010985#logs

error due to this change
apps/testing: Move irtest/sensortest/resmonitor/monkey to apps/system #2976

Impact

Impact on user: NO.

Impact on build: YES. fix the build with Make

Impact on hardware: NO

Impact on documentation: NO

Impact on security: NO

Impact on compatibility: NO

Testing

Build it with:
./tools/configure.sh -l sim:rc
make -j

Without this PR

With this PR

[simbit18]

@lupyuen I ran a test on github and again, of course, the same error comes up

https://github.com/simbit18/nuttx_test_pr/actions/runs/13436257100/job/37539227553#logs

Also here as with sim:quickjs #2993

on the NuttX repository it is built correctly because on Ubuntu it uses Cmake

====================================================================================
Cmake in present: sim/rc
Configuration/Tool: sim/rc
2025-02-20 09:08:52
------------------------------------------------------------------------------------
  Cleaning...
  Configuring...
  Select HOST_LINUX=y
  Select HOST_X86_64=y
  Building NuttX...
====================================================================================

https://github.com/apache/nuttx/actions/runs/13431136480/job/37523189277#logs

[cederom]

Thank you @simbit18 :-)