Inconsistent execution environments make using a path-as-define
difficult, so switch to an embedded file.
Also fixes the lldb launch so that passing tests run cleanly, and adds
TEST_TARGET to gdb (but without testing there). I'm dropping `sourceMap`
because it's not handled quite correctly (also not great to be trying to
pass source mappings in two different ways), and `env` didn't seem to be
working as intended either; maybe specifying `initCommands` causes other
things to not be evaluated. But the straight `initCommands` looks like
it's working. I used lldb to validate execution of these changes.
```
Running initCommands:
(lldb) command script import external/+llvm_project+llvm-project/llvm/utils/lldbDataFormatters.py
(lldb) settings set target.source-map "." "/usr/local/google/home/jperkins/dev/carbon-lang"
(lldb) settings set target.source-map "/proc/self/cwd" "/usr/local/google/home/jperkins/dev/carbon-lang"
(lldb) env TEST_TARGET=//toolchain/testing:file_test
(lldb) env TEST_TMPDIR=/tmp
Running tests with 128 thread(s)
.
Done!
Note: Google Test filter = ToolchainFileTest.toolchain/check/testdata/const/collapse.carbon
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from ToolchainFileTest
[ RUN ] ToolchainFileTest.toolchain/check/testdata/const/collapse.carbon
[ OK ] ToolchainFileTest.toolchain/check/testdata/const/collapse.carbon (0 ms)
[----------] 1 test from ToolchainFileTest (0 ms total)
[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (1 ms total)
[ PASSED ] 1 test.
Process 3869310 exited with status = 0 (0x00000000)
```
Thomas Köppe
Jon Ross-Perkins