fix generic_address_space, command_buffer_event_sync, test_compiler and images/test_1D_buffer

[franz]

fixes several issues:

• generic_address_space test: add check for program scope variables. The test uses both generic AS and program-scope variables, however it only checked the generic-AS presence in clGetDeviceInfo.
• compiler/test_compiler_defines_for_extensions.cpp: add cl_khr_command_buffer_multi_device to the list of recognized extensions
• command_buffer_event_sync.cpp: add delays for testing clSetEventCallback - according to specification, these can be executed asynchronously (in a separate thread) by the OpenCL implementation, hence the event callback is not quaranteed to be called before clFinish() returns. Existing test events/test_callbacks.cpp also waits for callback with loops of usleep.
• images/kernel_image_methods/test_1D_buffer.cpp: fix allocation size being too small for the 1D buffer backing the image

[franz]

The code check is complaining about the extension list, however i think the existing formatting is more readable.
Edit: added // clang-format off/on

[pjaaskel]

This and #1840 are blocking a successful conformance run for PoCL on x86_64 and Level Zero.

[franz]

Dropped the conflicting commit & rebased onto main.

[bashbaug]

Merging as discussed in the October 1st teleconference.