tests/run-tests.sh: Cleanup: Rename internal variable
tests/run-tests.sh: Cleanup: Rename internal variable

"test_installation" is ambiguous: Is it a verb (test the installation)
or a noun (the test installation)? If it's a verb, does it mean to test
the installation process or to test the version that's already
installed? (It's the latter.)

stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester