fix: Isolate inode cache file in tests

fix: Isolate inode cache file in tests

All test suites use the default temporary directory location
/run/user/<UID>/ccache-tmp when possible, which means that the inode
cache file is shared between all test suites. This is problematic when
running test suites in parallel since one test suite may run “ccache -C”
(which removes the inode cache file) between two compilations in the
inode_cache suite, thus making the second compilation not behave as
expected, failing the test.

Fix this by putting the temporary directory inside the test-specific
ccache directory instead of using the default global location.

Fixes #1045.