Fix issue 17375: Use OPTIONAL_PIC in the test suite

[Burgos]

Test suite didn't compile the executable with fPIC or with -cov no
matter if that was set in the top makefile (since it is a) overriding
DFLAGS, and b) because OPTIONAL_PIC was not exported from the main make
file to sub make files).

This makes test suite compile executables with the fPIC if requested
with PIC. Incidentally, this fixes a problem compiling shared test suite
on arch linux with binutils greater than v2.28, since the executable
itself will be compiled with -fPIC, not just shared library.

N.B.: the -cov flag is present, but not exported and used, because
some shared library tests are segfaulting when compiled with -cov.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Description
✗	17375	colliding modules detected with binutils 2.28 linker and shared libraries

[Burgos]

Edit: removed "Fix issue" annotation, since it just worksaround it so that testsuite may work.

[Burgos]

Added -dip1000 flag to test-suite while I was there as well.

[Burgos]

Bah, now I found out #1721 which is staled from December. :grumpy_cat:. Closing as duplicate, will reopen a new one with the -dip1000 now.