Test `run-test.sh` fails if bwrap is absent

Unfortunately, log doesn't give much information, but this is a consequence of the addition of set -e in commit b5bcf61f.

PASS: test-bz131804.exe
PASS: test-bz96676.exe
PASS: test-bz89617.exe
FAIL: run-test.sh
PASS: test-name-parse.exe
PASS: test-bz1744377.exe
PASS: test-issue180.exe
PASS: test-issue107.exe
PASS: test-bz106632.exe

$ cat run-test.log
FAIL run-test.sh (exit status: 1)

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