Simplify rdmd_test flag for specifying default compiler

Since it seems there are some use-cases where it is unavoidable to run
`rdmd_test` directly, it's nice if we can reduce the amount of typing
required to do so.

Besides the rdmd executable itself, the only other obligatory argument
to the test suite is the default compiler expected to be used by rdmd.
This patch shortens the long option to `--default-compiler` and allows
an equivalent single-character `-D` option, so that usage can now be of
the form:

    rdmd_test -D <default-compiler> <path-to-rdmd-executable>

posix.mak

@@ -121,7 +121,7 @@ endif
 
 test_rdmd: $(ROOT)/rdmd_test $(RDMD_TEST_EXECUTABLE)
 	$< $(RDMD_TEST_EXECUTABLE) -m$(MODEL) \
-	   --rdmd-default-compiler=$(RDMD_TEST_DEFAULT_COMPILER) \
+	   --default-compiler=$(RDMD_TEST_DEFAULT_COMPILER) \
 	   --test-compilers=$(RDMD_TEST_COMPILERS) \
 	   $(VERBOSE_RDMD_TEST_FLAGS)
 	$(DMD) $(DFLAGS) -unittest -main -run rdmd.d

rdmd_test.d

@@ -55,7 +55,7 @@ int main(string[] args)
     string testCompilerList; // e.g. "ldmd2,gdmd" (comma-separated list of compiler names)
 
     auto helpInfo = getopt(args,
-        "rdmd-default-compiler", "[REQUIRED] default D compiler used by rdmd executable", &defaultCompiler,
+        "D|default-compiler", "[REQUIRED] default D compiler used by rdmd executable", &defaultCompiler,
         "concurrency", "whether to perform the concurrency test cases", &concurrencyTest,
         "m|model", "architecture to run the tests for [32 or 64]", &model,
         "test-compilers", "comma-separated list of D compilers to test with rdmd", &testCompilerList,