Tool: Use TerminalNotifier in singletest

[theotherjimmy]

Description

According to #6914 comment thread, singletest did not get updated with the rest
of the tools in the notifier refactor. I thought this was because it's a
deprecated tool without users. We have a user, so we have a fix.

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[jeromecoutant]

Please include this PR before next OS2 label
Thx

[theotherjimmy]

@jeromecoutant So this resolves your errors?

[jeromecoutant]

Yes, I can make OS2 tests again
Thx

[0xc0170]

Not certain if I left a comment here , github does not show it at the moment. @cmonr made one more additional commit to fix mbed 2 build, do we need to add it here or all good?

[cmonr]

@0xc0170 See: #7132 (comment)

Tl;dr: Nope.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2257
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7124/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 2046
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7124/2046

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1882
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7124/