CI:add TEST_DIR removal to self-hosted workflow

[tszczyp]

Added secret variable for TEST_DIR

Added step to remove TEST_DIR at the end of job

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This change is 

[codecov-commenter]

Codecov Report

Merging #209 (6f5d6ae) into master (ab42b71) will increase coverage by 0.18%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   91.54%   91.72%   +0.18%     
==========================================
  Files          12       12              
  Lines         828      834       +6     
==========================================
+ Hits          758      765       +7     
+ Misses         70       69       -1     

Flag	Coverage Δ
tests_clang_debug_cpp17	92.67% <ø> (-0.07%)	:arrow_down:
tests_gcc_debug_cpp17	89.49% <ø> (+0.99%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/libpmemstream.c	88.92% <0.00%> (-0.12%)	:arrow_down:
src/region.c	93.57% <0.00%> (+1.42%)	:arrow_up:

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[karczex]

.github/workflows/self_hosted.yml line 15 at r3 (raw file):

        os: [[self-hosted, ubuntu]]
    env:
      PMEMSTREAM_TEST_DIR: ${{ secrets.SELF_HOSTED_RUNNERS_TEST_DIR }}/${{ github.job }}

It seems secrets doesn't work on PRs :/
So you have to change to partially hardcoded `/mnt/pmem0/${{github.job }}

Code quote:

 ${{ secrets.SELF_HOSTED_RUNNERS_TEST_DIR }}