Switch to custom unit test build.

For some reason, as evidenced by
#2, projects that depend on dxml
that then do their own unit test builds get linker errors due to stuff
in the unit tests. Those tests really shouldn't even be being built when
dxml is used as a dependency anyway, but for whatever reason, dub builds
all dependencies with the unittest target when a project is built with
the unittest target.

To fix that, dxml now has a separate "doTests" build type which defines
the version dxmlTests, and all unittest blocks are versioned with
dxmlTests. So, unfortunately, dub test will not run the tests anymore,
but we're stuck it seems.

dub.json

@@ -4,5 +4,13 @@
     "description": "An XML parser and writer written in D.",
     "copyright": "Copyright © 2017 - 2018, Jonathan M Davis",
     "license": "BSL-1.0",
-    "targetType": "library"
+    "targetType": "library",
+    "buildTypes":
+    {
+        "doTests":
+        {
+            "buildOptions": ["unittests", "debugMode", "debugInfo"],
+            "versions": ["dxmlTests"]
+        }
+    }
 }

source/dxml/dom.d

@@ -26,7 +26,7 @@
 module dxml.dom;
 
 ///
-unittest
+version(dxmlTests) unittest
 {
     auto xml = "<!-- comment -->\n" ~
                "<root>\n" ~
@@ -812,11 +812,11 @@ private struct DOMCompileTests
     @property typeof(this) save() @safe pure nothrow @nogc { assert(0); }
 }
 
-version(unittest)
+version(dxmlTests)
     DOMEntity!DOMCompileTests _domTests;
 
 /// parseDOM with the dault Config and a range of characters
-@safe unittest
+version(dxmlTests) @safe unittest
 {
     import std.range.primitives;
 
@@ -864,7 +864,7 @@ version(unittest)
 }
 
 /// parseDOM with simpleXML and a range of characters
-unittest
+version(dxmlTests) unittest
 {
     auto xml = "<root>\n" ~
                "    <!-- no comment -->\n" ~
@@ -908,7 +908,7 @@ unittest
 }
 
 /// parseDOM with simpleXML and an EntityRange
-unittest
+version(dxmlTests) unittest
 {
     import dxml.parser : parseXML;
 
@@ -957,7 +957,7 @@ unittest
 }
 
 /// parseDOM with an EntityRange which is not at the start of the document.
-unittest
+version(dxmlTests) unittest
 {
     import dxml.parser : parseXML, skipToPath;
 
@@ -1061,7 +1061,7 @@ void _parseDOM(ER, DE)(ref ER range, ref DE parent, ER.SliceOfR[] path = null)
     parent._children = children.data;
 }
 
-unittest
+version(dxmlTests) unittest
 {
     import std.algorithm.comparison : equal;
     import dxml.internal : testRangeFuncs;

source/dxml/internal.d

@@ -36,7 +36,7 @@ bool equalCU(R)(R range, string text)
     return range.empty;
 }
 
-unittest
+version(dxmlTests) unittest
 {
     import std.algorithm : filter;
     import std.meta : AliasSeq;
@@ -87,7 +87,7 @@ pragma(inline, true) auto stripBCU(Orig, R)(R range)
         return range;
 }
 
-unittest
+version(dxmlTests) unittest
 {
     import std.algorithm : equal, filter;
     import std.range : takeExactly;
@@ -123,7 +123,7 @@ pragma(inline, true) auto checkedSave(R)(ref R range)
 // Unit test helpers
 //------------------------------------------------------------------------------
 
-version(unittest):
+version(dxmlTests):
 
 struct FwdCharRange(C)
 {