Base.runtests: unset JULIA_PROJECT and JULIA_LOAD_PATH

[DilumAluthge]

Closes #43574

This is a follow-up to #42358.

[DilumAluthge]

@mkitti Can you test this out locally and confirm that it fixes your use case?

[mkitti]

It works with 91b8f0c.

$ JULIA_PROJECT="@foobar" JULIA_LOAD_PATH="@:@foobar:@stdlib" usr/bin/julia -e 'Base.runtests(["loading","cmdlineargs"])'
Test    (Worker) | Time (s) | GC (s) | GC % | Alloc (MB) | RSS (MB)
loading      (2) |        started at 2021-12-28T15:38:52.203
cmdlineargs  (3) |        started at 2021-12-28T15:38:52.305
      From worker 2:	┌ Error: active project callback #19 failed
      From worker 2:	└ @ Base initdefs.jl:325
      From worker 2:	WARNING: replacing module Foo.
loading      (2) |    25.21 |   0.59 |  2.4 |    1959.53 |   367.73
cmdlineargs  (3) |   228.90 |   0.45 |  0.2 |     819.24 |   343.24

Test Summary: |   Pass  Broken   Total     Time
  Overall     | 145913       3  145916  3m52.4s
    SUCCESS

$ JULIA_PROJECT="@foobar" JULIA_LOAD_PATH="@:@foobar:@stdlib" usr/bin/julia -e 'using InteractiveUtils; versioninfo()'
Julia Version 1.8.0-DEV.1183
Commit 91b8f0c1de* (2021-12-28 10:31 UTC)
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: AMD FX(tm)-8350 Eight-Core Processor
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-12.0.1 (ORCJIT, bdver1)
Environment:
  JULIA_LOAD_PATH = @:@foobar:@stdlib
  JULIA_PROJECT = @foobar
  JULIA_CPU_TARGET = generic;native

[mkitti]

While I do think this is simpler, I still do think we should annotate the appropriate tests that they are dependent on having default values for JULIA_PROJECT and JULIA_LOAD_PATH. I also note that this make it harder to test the effects of changing these variables on all the other tests where they should have no effect.