Today I had the chance to bootstrap and runthe testsuite for trunk on
x86_64-w64-mingw32. Bootstrap is done with all supported languages
enabled including "D".

Testsuite results can be found here:
https://gcc.gnu.org/ml/gcc-testresults/2019-04/msg01795.html

Complete logs here:
https://cloud.emrich-ebersheim.de/index.php/s/g9D245XdCW6GD5W

There's are two issues with the gdc testsuite.
1. The failed tests are printed twenty times to the test_summary.txt file.

2. couldn't open "gdc.test/compilable/99bottles.d"
ERROR: tcl error sourcing
/opt/devel/gnu/src/gcc-mingw-w64/gcc-9.0.1/gcc/testsuite/gdc.test/gdc-test.exp.
ERROR: couldn't open "gdc.test/compilable/99bottles.d": no such file or
directory
    while executing
"open $file r"
    (procedure "grep" line 19)
    invoked from within
"grep $prog "{\[ \t\]\+dg-\[-a-z\]\+\[ \t\]\+.*\[ \t\]\+}" line"
    (procedure "dg-get-options" line 4)
    invoked from within
"dg-get-options $prog"
    (procedure "saved-dg-test" line 75)
    invoked from within
"saved-dg-test gdc.test/compilable/99bottles.d { }
-I/opt/devel/gnu/src/gcc-mingw-w64/gcc-9.0.1/gcc/testsuite/gdc.test/compilable"
    ("eval" body line 1)
    invoked from within
"eval saved-dg-test $args "
    (procedure "dg-test" line 4)
    invoked from within
"dg-test $test "$flags $flags_t" ${default-extra-flags}"
    (procedure "gdc-dg-runtest" line 23)
    invoked from within
"gdc-dg-runtest $filename $flags $imports"
    (procedure "gdc-do-test" line 86)
    invoked from within
"gdc-do-test"
    (file
"/opt/devel/gnu/src/gcc-mingw-w64/gcc-9.0.1/gcc/testsuite/gdc.test/gdc-test.exp"
line 465)
    invoked from within
"source
/opt/devel/gnu/src/gcc-mingw-w64/gcc-9.0.1/gcc/testsuite/gdc.test/gdc-test.exp"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 source
/opt/devel/gnu/src/gcc-mingw-w64/gcc-9.0.1/gcc/testsuite/gdc.test/gdc-test.exp"
    invoked from within
"catch "uplevel #0 source $test_file_name""

Hope this is of any help