chore: Stop swallowing errors in .sh tests

The Bash shell scripts ignored errors in all but the last line.
This commit adds `set -eo pipefail` to fix the issue:

`set -e` causes the script to exit as soon as any command exits with a
non-zero exit code. When pipes are used, this flag only applies to the
last command in a pipe. The `set -o pipefail` option ensures that the
script is also exited with a non-zero exit code when any command in the
pipe fails.

test/run-browsers-smoketests.sh

@@ -1,4 +1,6 @@
 #!/bin/bash
+set -eo pipefail
+
 echo ""
 echo "Test webextension-polyfill on real browsers"
 echo "==========================================="

test/run-module-bundlers-smoketests.sh

@@ -1,4 +1,6 @@
 #!/bin/bash
+set -eo pipefail
+
 echo ""
 echo "Test webextension-polyfill bundled with webpack"
 echo "==============================================="