Fix test ClassLoader issue

[stuartwdouglas]

This prevents the base runtime CL from
loading deployment classes.

Fixes #16810
Fixes #16804

[Ladicek]

This basically uses the approach I proposed in https://quarkusio.zulipchat.com/#narrow/stream/187038-dev/topic/Avro.20vs.2E.20continuous.20testing, where the base CL would throw CNFE for application classes? Nice! I didn't know it would be this easy, thanks :-)

[quarkus-bot]

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Failing Jobs - Building f1924d4

Status	Name	Step	Test failures	Logs	Raw logs
⌛	Initial JDK 11 Build	Build	⚠️ Check →	Logs	Raw logs
🚫	Devtools Tests - JDK ${{ matrix.java.name }}
🚫	Devtools Tests - JDK 11 Windows
🚫	Gradle Tests - JDK 11 ${{ matrix.os.family }}
🚫	JVM Tests - JDK ${{ matrix.java.name }}
🚫	JVM Tests - JDK 11 Windows
🚫	Maven Tests - JDK ${{ matrix.java.name }}
🚫	Maven Tests - JDK 11 Windows
🚫	MicroProfile TCKs Tests
🚫	Native Tests - ${{ matrix.category }}
🚫	Native Tests - Windows - ${{ matrix.category }}

[stuartwdouglas]

When I added the flat-class-path I was running out of time to get CT into Alpha1, and did not realize that it would delegate to the TCCL if the original CL failed to look it up.

[gsmet]

@stuartwdouglas the TCK failures look suspicious.