[Tests] fix 4 test failures

- `nvm-exec` test: expect "Found .nvmrc" message in output, since `nvm_rc_version` now outputs it to stdout via fd 3 redirection (ef162036)
- `nvm_install_binary_nosource`: fix exit code capture by running the command directly instead of inside a subshell with `echo $?` (05d78477)
- `nvm_iojs_version_has_solaris_binary`: bare versions like `v3.3.1` (without `iojs-` prefix) are node versions and should be rejected. The old tests relied on the buggy comparison that let them through (53e6244a)
- `nvm_get_arch_unofficial`: copy `uname` into the chroot. The old test passed only because the unconditional `NVM_ARCH=x64-musl` masked the missing binary, but the `case` fix now requires a real arch to match (39e71eab)

test/fast/Unit tests/nvm_install_binary_nosource

@@ -23,7 +23,7 @@ if [ "${OUTPUT#*$EXPECTED_OUTPUT}" = "${OUTPUT}" ]; then
   die "No source binary flag is active and should have returned >${EXPECTED_OUTPUT}<. Instead it returned >${OUTPUT}<"
 fi
 
-EXIT_CODE="$(nvm_install_binary node std 8.0.0 1 2>/dev/null; echo $?)"
+nvm_install_binary node std 8.0.0 1 >/dev/null 2>&1; EXIT_CODE=$?
 [ "${EXIT_CODE}" = "2" ] || die "Expected exit code 2 when nosource=1 and binary fails, got ${EXIT_CODE}"
 
 # Unit test to check if the function errors out when the flag is set