Fix case when jshint is not available

[fredden]

When I run the test suite on a system where jshint is not installed, I get the following error:

............................................................. 1159 / 1729 ( 67%)
............................................................. 1220 / 1729 ( 70%)
............................................................. 1281 / 1729 ( 74%)
............................................................. 1342 / 1729 ( 77%)
............................................................. 1403 / 1729 ( 81%)
............................................................. 1464 / 1729 ( 84%)
.........S................................................... 1525 / 1729 ( 88%)
......SSjs: Couldn't read source file "":  (No such file or directory).
F

80 sniff test files generated 242 unique error codes; 126 were fixable (52%)

Time: 2.03 seconds, Memory: 36.00 MB

There was 1 failure:

1) PHP_CodeSniffer\Standards\Generic\Tests\Debug\JSHintUnitTest::testSniff
[LINE 3] Expected 2 warning(s) in JSHintUnitTest.js but found 0 warning(s).

/path/to/repository/squizlabs-php-codesniffer/tests/Standards/AbstractSniffUnitTest.php:206
/path/to/repository/squizlabs-php-codesniffer/tests/TestSuite7.php:28

FAILURES!
Tests: 1534, Assertions: 9422, Failures: 1, Skipped: 3.

Note the error (js: Couldn't read source file "": (No such file or directory).) part-way through the test-suite output.
The command being run is /usr/bin/rhino "" '/path/to/repository/squizlabs-php-codesniffer/src/Standards/Generic/Tests/Debug/JSHintUnitTest.js'

This pull request fixes this by skipping the test when jshint is not available on the system.

[jrfnl]

Makes sense to me.

Note: This is a follow up on PR #2362, which already improved the logic in the sniff regarding the external tools, but didn't take into account a potential situation where Rhino is available, but jshint is not.

[jrfnl]

FYI: this fix is included in today's PHP_CodeSniffer 3.8.0 release.

As per #3932, development on PHP_CodeSniffer will continue in the PHPCSStandards/PHP_CodeSniffer repository. If you want to stay informed, you may want to start "watching" that repo (or watching releases from that repo).