[release/dev18.0] Fix build task integration tests

[jjonescz]

Backport of #81128.
Closes #81246.

[jjonescz]

/backport to release/dev18.3

[github-actions]

Started backporting to release/dev18.3 (link to workflow run)

[github-actions]

@jjonescz backporting to release/dev18.3 failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: [release/dev18.0] Fix build task integration tests
Applying: Unskip tests
Using index info to reconstruct a base tree...
M	src/Compilers/Core/MSBuildTaskTests/TestUtilities/IntegrationTestBase.cs
Falling back to patching base and 3-way merge...
Auto-merging src/Compilers/Core/MSBuildTaskTests/TestUtilities/IntegrationTestBase.cs
CONFLICT (content): Merge conflict in src/Compilers/Core/MSBuildTaskTests/TestUtilities/IntegrationTestBase.cs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0002 Unskip tests
Error: The process '/usr/bin/git' failed with exit code 128

Link to workflow output