Remove test list gpr [Don't merge]

[Fabien-Chouteau]

While trying #2082 I realized that to build an executable with gprbuild you can just pass the source file on the command line:

# gprbuild main.adb

So I did a quick check and it turns out you can do this even if main.adb is not listed in for Main use (); in the GPR file.

Using this feature, we can get rid of the system that produces a GPR file with the list of tests to build.

Advantages:

• Less code
• The test crate is "buildable" right away (see below). No need to run Alire to create the missing GPR file
• The less GPR files there are, the happier I am ^^

Drawbacks:

• Building the tests crate without passing args will not build any executable...

So it's not clear whether or not this a positive change. I just wanted to record this option.

Ping @AldanTanneo

[AldanTanneo]

This doesn't seem to work for nested tests right now. I like the idea but I don't know if it's very feasible to pass a long testsuite entirely on the command line. My first attempt at this as you know was through environment variables...