Today, the test scenario "ACTCTX_FLAG_HMODULE_VALID but hModule if not set" is broken and unreliable. This problem is not evident in WineHQ batch test runs; rather, the test failure seems to only be triggered when the kernel32:actctx test is run in isolation.
When the flag ACTCTX_FLAG_HMODULE_VALID is specified in ACTCTX but hModule is set to NULL, CreateActCtxW() may encounter different failure modes depending on the environment and/or the test executable file. Error codes observed so far include ERROR_SXS_CANT_GEN_ACTCTX and ERROR_SXS_MANIFEST_TOO_BIG.
It appears that the inconsistent failure was caused by Windows trying to interpret the main executable file of the current process as an XML manifest file. Note that a valid PE executable that starts with the "MZ" signature is not a valid XML file; furthermore, the test executable may simply be too large for a single manifest file.
Fix this by changing the FullDllName of the main executable module's LDR_DATA_TABLE_ENTRY to the pathname of a temporary manifest file (valid or invalid) before testing. The testing is performed in a child process, since it deliberately "corrupts" the process state.