fmf: Fix node_modules/ handling for tarballs
fmf: Fix node_modules/ handling for tarballs

Commit 121e770ab9a regressed running tests from tarballs: As there is no
.git directory, tools/node_modules fails to run. So add a check for .git.

We can also simplify this: Since commit c00710475274a, our tarballs
include the NPM modules necessary for testing, so we never need to run
npm.

martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit