testit 1.1.2

• test_pkg() no longer reinstalls the package when it is run under covr (detected via the R_COVR environment variable), and installs the package at most once per session so that repeated test_pkg() calls (e.g. a CRAN suite plus a CI-only suite in tests/test-all.R) reuse the first install. Previously the reinstall clobbered covr's instrumented build, breaking coverage with errors such as undefined symbol: __gcov_merge_add (#28).