Repository: opencv
Issue: GAPI test fails on RISC-V
Labels: test,category: g-api / gapi

OpenCV version: 4.5.2-pre, Last reproduced: ad333e0

The test fails from time to time on cross build for RISC-V + QEMU: https://build.opencv.org.cn/job/nightly-master/job/ubuntu-20.04-riscv64/119/

Error message:
```
unknown file
C++ exception with description "OpenCV(4.5.2-pre) /home/opencv-cn/slave/workspace/nightly-master/ubuntu-20.04-riscv64/opencv/modules/gapi/include/opencv2/gapi/streaming/cap.hpp:70: error: (-215:Assertion failed) false && "Couldn't grab the very first frame" in function 'prep'
" thrown in the test body.
```
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##### System information (version)
<!-- Example
- OpenCV => 4.5.2
- Operating System / Platform => RISC-V
- Compiler => Visual Studio 2017
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- OpenCV => :grey_question:
- Operating System / Platform => :grey_question:
- Compiler => :grey_question:

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##### Steps to reproduce

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