Nant how to list all the targets in the current project

You can define properties in one target, and use their values in the other... For example, you can define

<target name="run-unit-tests">
   <property name="test.executable" value="tools\nunit\nunit-console.exe"/>
   <property name="test.extratestargs" value="foo,bar,baz"/>
   <call target="do-unit-tests"/>
</target>

<target name="run-unit-tests-teamcity">
   <property name="test.executable" value="${teamcity.dotnet.nunitlauncher}"/>         
   <property name="test.extrtestargs" value="foo,baz,quux,xyzzy"/>
   <call target="do-unit-tests"/>
</target>

<target name="do-unit-test-coverage">
   <property name="test.args" value="${test.assemblies} ${test.extratestargs} <!--snip -->" />
   <ncover <!--snip -->
           commandLineArgs="${test.args}" >
   <!--snip-->
   </ncover>
</target>

Or if you need them to be structured completely differently, not just have some different values, take advantage of the fact that the property substitution is delayed:

<?xml version="1.0"?>

<project name="nanttest">

        <target name="run-unit-tests">
           <property name="test.executable" value="tools\nunit\nunit-console.exe"/>
           <property name="test.args" value="foo bar -assembly ${test.assemblies} baz" dynamic="true"/>
           <call target="do-unit-test"/>
        </target>

        <target name="run-unit-tests-teamcity">
           <property name="test.executable" value="${teamcity.dotnet.nunitlauncher}"/>
           <property name="test.args" value="foo,baz,quux /a:${test.assemblies} xyzzy" dynamic="true"/>
           <call target="do-unit-test"/>
        </target>

        <target name="do-unit-test-coverage">
           <echo message="test.executable = ${test.executable}, test.args = ${test.args}" />
        </target>

        <target name="do-unit-test">
           <property name="test.assemblies" value="MyProject.dll"/>
           <call target="do-unit-test-coverage" />
        </target>


</project>

user@host:/tmp/anttest$ nant run-unit-tests
[...snip...]
run-unit-tests:
do-unit-test:
do-unit-test-coverage:
     [echo] test.executable = tools\nunit\nunit-console.exe, test.args = foo bar -assembly MyProject.dll baz
BUILD SUCCEEDED
Total time: 0 seconds.

user@host:/tmp/anttest$ nant -D:teamcity.dotnet.nunitlauncher=nunitlauncher run-unit-tests-teamcity
[...snip...]
run-unit-tests-teamcity:
do-unit-test:
do-unit-test-coverage:
     [echo] test.executable = nunitlauncher, test.args = foo,baz,quux /a:MyProject.dll xyzzy
BUILD SUCCEEDED
Total time: 0 seconds.

If you really, really just need to know if you're running in TeamCity, then this should help:

<target name="run-unit-tests-teamcity">
   <property name="test.executable" value="${teamcity.dotnet.nunitlauncher}"/>         
   <property name="running.in.teamcity" value="true"/>
   <call target="do-unit-tests"/>
</target>

My recommendation is to always place the build file at solution level. Then all relative paths in the build file will be equal to that of the solution.